Hi,
Lots of good advice here. If you’re comfortable with your LHT long distances then I would stick with it, unless you’ve got the time to learn what your comfortable position might be on a lighter bike. Based on what you have already said about your plans, with some modest weight loss (but only if you want to do that) and riding more miles I reckon you will be fine!
I rode it last year and would say that, in comparative terms, it really isn’t that difficult for 100 miles – it’s quite a flat course for much of the route (e.g. barring Sawyer’s Hill in Richmond Park the first 40 miles are all virtually flat). In the Surrey Hills, the route misses out all of the properly steep climbs on the Downs (like Whitedown, Coldharbour & Ranmore Common) and the crowds are very encouraging!
The hardest bit on the route is Leith Hill and while it’s noticeably a climb it isn’t very long or really very steep (it’s less than 10%, compared to Whitedown which is 18%). Box Hill is easier than Leith Hill and after that there isn’t too much uphill left to do (the biggest remaining climbs if you can call them that, is the drag up Coombe Hill from Norbiton and I think one of the bits around Wimbledon Park). Put it this way - I had a gearing problem last year which meant I couldn't use my granny ring at all and Leith Hill was the only place I would have used it had I had the option.
With missing out many of the hills and the narrower roads, most of the route takes place on main roads in good repair so I wouldn’t worry too much about the road surface (it’s not great on parts of the A25 but that’s as bad as I recall it being). Also you can sit in a big bunch for much of the time and get dragged along – sure you’ll probably drop off the back when the gradient climbs but you won’t be the only one and there will be another group along in a bit every time!
The closed roads help too in that there is a lot of road to use which makes descending a lot of fun, even for nervous descenders like me.
Lastly, unless you desperately need to stop there, try and avoid the first feed station at Hampton Court as you’ll queue for ages for the loos - there are plenty of other places to stop later on. From memory, you’re offered a stopping opportunity roughly every 20 miles or so.