It's not mad at all, but just beware of losing motivation after a big event along the way.
My 2016 was supposed to be Brighton Marathon (mid-April), Mersey Roads 24h TT (July 23/24) and then Ironman Wales in September.
My training wasn't going too badly. I only work 4 days a week so one day I'd drop my daughter off at school at 9am, do a 4k-5k swim and then go out on the bike for 3 hours, or a bit shorter and finish with a 5-10k run, home and showered and back to school to pick her up at 3pm.
This was alongside one cycle commute (to/from work) a week. A few runs (parkrun as a tempo run on a Saturday, a midweek interval session, Friday 11k run into work, Sunday long run). Also a quick Wednesday swim (whilst my daughter was having a lesson) to do some swim intervals (rather than just one long steady session a week). One clear day off a week.
The marathon (Brighton in April, my first) went ok. Legs emptied at 19 miles and there was 5k of walking followed by 5k of walk/jog (900m jog, 100m walk) but I finished. Was hoping for 4h30, and would have gone through halfway in 2h15 if I hadn't wasted 5 minutes stopping for a wee that I didn't need. The poor finish gave me a 5h07 which wasn't bad given my BMI was nearly 28.
I took a week off any exercise after the marathon, but found it easy to get going again after that week off.
I'd also done a 100 and a 200 km Audax, focusing on stopping as little as possible and pushing a bit harder. Was happy to do the 200 in 9h10 moving and only 45m stopped, which is very unlike me.
Weight was edging down and I think I got to 82kg at the lowest. Continued the running maxing out at 2h for the long run (I'd read somewhere that was good enough for IM given my weightiness). Local HM around Wimbledon Common in May went really well (2:06:48). As the weather warmed up I did a few trips to Shepperton Lake for some open water (wetsuit) swimming. 1h cycle each way and a 2.5km to 4km swim.
On top of all of this was two separate 5-a-side games, one 40 minutes (in a league so higher intensity) and another hour long game with friends. The stop/start jog/walk/sprint/run/walk/sprint/walk nature really helped with the fitness and took away from the monotony of the rest of the exercise which was very straight-line / linear.
MR24 was a bit of fun, I'd not really done enough cycling (I later worked out I'd done a sum total of 58h on the bike Jan 1 to July 23) which is way less than I'd hoped. Given this I was hoping for 500km but fell short at ~475km. I only had 19h50m moving time, less than I'd hoped for (stomach issues forced me off for a 90 minute break where I tried to sleep...). I went through the IronMan cycle distance (180km) in 7h15 (elapsed) and was very happy with that, especially as I was riding fixed and within myself enough to think about riding for 24h.
Ideas beginning to form of a 1h15 swim, 7h15 cycle, 15 minutes T1+T2 = 8h45 and then a 5h marathon = 13h45 IM. I would have been deleriously happy with that.
( Targeting the 24h properly I'd go for 600km (or maybe even 400miles) but I'd probably have to pass on triathlon or marathon goals for that. )
What the 24 did do was wipe out my motivation for any form of training. Combined with the summer holidays I didn't do anything meaningful for another 3 and a half weeks. I just could not be arsed, even though I knew I should be doing things as IM Wales was looming...
But I was still in OK shape, as confirmed by a few runs and swims. A week before IM Wales I got a sinus infection, was prescribed antibiotics and advised that a 1h15m sea swim was probably not the best thing to do given the state of my sinus/chest. Since then I've had a series of coughs/colds that never seem to clear up but haven't stopped me completely in my tracks. In the last 5 months I've slowly put the weight back on whilst watching the weeks to Brighton tick down. 11 weeks to go and I've got to put the effort in now and reassess with about 6 weeks to go.
So, there's no reason why you can't do a marathon and LEL. Just keep an eye on things and not let them spiral off.
I didn't do everything I wanted as well was I wanted as I just ran out of time to train, and hadn't really thought about the timings very well. I don't want to miss out on family time and the school holidays removed the bulk of my training time exactly at the wrong time (I was supposed to be ramping up to ~15h a week in the 8-4 weeks away from IM Wales). Piss poor planning and foresight really. The primary reason I'd moved down to 4 days a week was to cover childcare during the holidays, likewise the 2 weeks extra holiday I buy from work each year helps to cover the summer holidays.
Getting back on topic though...
General fitness aside, all the running I did made my cycling feel a tiny bit easier, but I found no benefit to my running from the silly levels of cycling I'd been doing (then or in the past). (As an aside, my skiing fitness and endurance benefited hugely from all of the running, but never has benefited from the cycling.)
Looking at the negatives, I didn't feel that either the running or cycling hindered the other in any way, but I'd definitely second the suggestion for doing lots of stretching. I need to keep on top of it otherwise the running really hampers what little flexibility I have.
Another activity (the so called 'cross training') helps too, be that swimming, walking or whatever. However I'm rather extreme in that I can go to the pool and knock out a 1h+ long swim without getting bored (years of swim coaching coupled with the Audax mentality sees to that).
Good luck with it, hopefully see you in Brighton (I'll be aiming for sub 4h30 if the next 3 months go well, sub 5h if average, and just finishing if I apply my usual levels of apathy...)
No LEL for me, just not enough time to get back into long long distance shape, and can't justify being away from my family for nearly a week during the summer holidays. Maybe in 4 or 8 years when she's older.
