I agree with most of Sheila's advice except the stuff about the choice of start times. I think it could discourage new riders from choosing the 90 hour start as they may think that it is for just for "Tourists" when it might actually be appropriate for them to have the maximum possible time available in case things go wrong or it's harder than they expect. Labelling riders as Tourists, Randonneurs an Vedettes isn't particularly helpful as all starters should in fact be Super Randonneurs. Choosing a 90 hour start doesn't mean you can't do a fast ride either. There are usually a few riders doing 40 something hour times from the 90 hour start and many finishing in under 70 hours. It is surprising how many people choose faster start times that put them under unnecessary time pressure and in many cases cause them to finish out of time or not at all. This happened a lot on the LEL where many 100 hour riders ran out of time, but could have finished in 117 hours.
My rule of thumb for choosing a start time for a first timer is to add 20 hours to the time that you think you can do a hilly 1200+km ride in and err on the side of caution.
If you realistically think you can get round in under 60 hours, you can choose 80, 84 or 90 hours
If you realistically think you can get round in under 64 hours, I would advise 84 or 90 hours
If you realistically think you can't get round in under 64 hours you might as well give yourself 90 hours
This is based on my own experience described briefly below and observation of thousands of other riders.
I tried riding in the 78 hour Vedettes group on my first attempt at PBP in 1987. The 90 hour starters had left at 0400 and the 84 hour starters had left at 1000, but then there was torrential rain before, during and after our 1600 start and a strong westerly wind. It had previously been very hot, melting some tar on the road and my front wheel slipped from under me on a descent after just 40 miles. My bike looked fine, but I needed patching up by the Red Cross at the first control. I carried on as far as I could, but by Loudeac my left knee had swollen up and was extremely painful so I packed rather than risking long term damage. I hardly had any time in hand due to the delays and headwind because the intermediate closing times are tighter for the vedettes. It was ironic because the previous year I had badly fractured my right knee cap in a car crash and doctors had warned that it might stop my long distance cycling, but this was my good knee. It turned out I had slightly bent my frame, forks, crank and bottom bracket spindle in the fall and this had caused the knee trouble rather than the fall itself. On reflection I had chosen the wrong start time. The vedette start is meant for very fast riders capable of getting round in well under 60 hours. I was no slouch. I'd already done the End to End as a 1400km BR in 73 hours 20 minutes that year and my last ride before PBP had been a 12 hour 300km, but that only put me at borderline fast enough for a vedette PBP, so the 84 hour start would have been more appropriate and a 90 hour start possibly even better as I was a newbie.
In 1991 there was a tailwind out and a strengthening headwind back. I had a few mechanical issues on the way to Brest, and as I arrived Sheila and Francis were just leaving. They had started with the vedettes a couple of hours before me, but I stopped for a three hour sleep at the control. My sleep that night proved a good investment because I passed Sheila and Francis the following night and only needed one more hours sleep to finish in well under 70 hours.
In 1995 I had a very intensive schedule of calendar audax rides from March to June, but then I had a prolapsed disk in my lower back so was unable to ride due to the intense pain, but I'd paid for PBP so I thought I'd still have a go. Noel Simpson and Pete Gifford were riding their "double bed" recumbant tandem trike and I had to ride to East Bridgford for a lift. I was late because I could hardly move, but luckily they waited for me and we caught the ferry from Portsmouth to Caen. I could keep up with the group on the two day ride to Paris, but was still in pain. Fortunately I had chosen the 90 hour start again so I had the minimum time pressure, but I was still worried whether I would be able to finish. Weirdly, with my first pedal stroke of the actual ride there was a clunk in my back and the pain instantly stopped. The conditions were great and I was flying again. I had long food stops at every control and had a five hour sleep in Carhaix on the way out, yet still got back in under 65 hours.
I had another big audax year in 1999 so was well prepared for the PBP and not injured this time. I wanted to repeat the strategy of 1995, but maybe go a bit faster. Things were going very well until on the way back approaching Loudeac I was pushed off the road by another British rider, causing us both to go down at high speed. I needed both arms and legs patching up and my front wheel truing at the next control and his rear derailleur was broken so he rode the last 300km in one gear. For me it was still under 66 hours, and for him a little over 67, so a bit dissappointing for both of us and maybe 4-5 hours longer than it might otherwise have taken.
By 2003 I was married with a daughter born late the previous year so my preparation had been minimal, just a qualifying series and a couple of 200s to keep my legs moving before the PBP. There was no doubt about choosing the 90 hour start again and this time I was with Rob Gray who I'd met on the Wessex Series in 2000. The ride from Caen with camping gear topped up my training and when I took all that gear off it felt quite easy. We had a great time, riding fast and stopping long, resulting in a time of under 67 hours. My wife didn't have such a great time as our daughter was teething whilst I was away!
2007 was the notoriously wet one and I now had two daughters. This time it was a family holiday and we were camping at Huttopia, Ramboullet. I wasn't very well prepared again and was on a new Dawes Audax Supreme. Despite the weather and my lack of fitness I still wanted to keep up my run of sub 70 hour rides, so I had a different strategy, carrying my food in a musette (bonk bag) at least to get me past the first few controls. I had planned to bin the bag half way round, but instead I refilled with Overstim bars bought at controls. I could't ride as fast and still had a total of six hours sleep, and my chainset fell off, but I still finished in 68:32 which I was more than happy with in those conditions.
I wasn't that fit in 2011 either and this time I planned to eat energy bars almost exclusively. I was carrying 48 at the start mostly in the fold down side pockets of my rack pack. I had almost new tyres on, but after 100 miles a bulge developed in my rear tyre and I was worried about it blowing out so I let a bit of air out. I had let somebody talk me out of carrying a spare tyre and was very tired as I hadn't slept the night before the ride, so ended up falling off as I was trying to feel the rear tyre when moving along and had to have a sleep in a haystack before the first control. I had to buy a new tyre, but still carried the faulty one and eventually reached Brest in about 39 hours so gave up on the idea of a sub 70 hour ride. I still wanted to get back to my family as soon as possible though and gradually I pulled back some of the lost time. By the last control I realised that if I really went for it I could still scrape round in under 70 hours and did so with 14 minutes to spare.
Since 1995 I had gradually been getting slower on the PBP and I considered this to be an inevitable consequence of the aging process, so surely my 2015 ride would be over 70 hours. One thing I had noticed though was that after my 5 hour sleep at Carhaix on the way out, quite a few 84 hour riders had caught me up and were carrying on very strongly. I wondered if I could be one of those riders, could I then ride through the first full night and get well on the way back from Brest before having to stop for sleep...With some trepidation I entered the 84 hour start in 2015 as I don't like giving up those potential 6 hours in case I might need them, but the gamble paid off and the ride went very well. I started at 0530 and gradually caught many of the 0515 and 0500 starters and sooner than expected began to catch 90 hour starters. Loudeac and Carhaix were very busy on the way out as I passed the bulge, but I was carrying my own food so didn't have to queue anywhere. After that I was mostly riding with vedettes who had stopped for sleep. I had also slept a couple of times, but only for a few minutes each. I got past Fougeres before I had a proper sleep for three hours and that wasn't really necessary, but I was fed up with night riding. It made the last 280km easier and I continued to catch more and more vedettes including many from group A, so had worked my way through the alphabet. My eventual time was 61:29 so my theory of getting slower with age was blown apart. The conditions in 2015 were perfect though, unless you were full value and got caught in the rain towards the end.