Keep going mate in January a 4 mile ride killed me soon was out to 8 that finished me off then out to 15 couldn't walk then 20 etc now. I fly through the 20 no stops feeling great. I'm up to 45 at the moment and planning a 50 for Sunday.
I was 20 stone 2 years ago lost weight by diet and gym and as I posted in another thread got a handle on a major problem got slim ish and bulked up but was still quite unfit then decided to do a triathlon next year to keep up motivation so learn to swim get on the bike and learn to run. I'm now shedding weight again and getting really fit it doesn't come easy but if it did everyone would do it only the dedicated and determined keep going and succeed.
Where I train we have a 24 stone man 4 months ago couldn't lap the field on the warm up run struggled to tyre flip or do any of the cardio in the classes he does now he's 20 stone run well and is b.....y strong.