I might have said this before but at the start of everybrun I wonder why I'm doing it. It's sore and uncomfortable. After about 3km I settle into it, find the rhythm and really enjoy it. After a few* more km I tyre and start to wonder why I'm doing it.

*When I'm not fit or just not on a good day, that is about 3km. When I'm really going well, that could be 10km or more. Getting fitter is about extending that middle bit.

I suspect the bit of running that I enjoyed recently may always be <2% of the time doing it. Always did seem a bit harsh and high-impact, though the woods and fields are a lot nicer to run on than tarmac.

All I'll add is just take a look at how many complete the London Marathon, and take a look at the ages, body shapes, etc., doing it.
It's all possible; you may not be the winner but you can certainly complete the course. Depends what you want.

James, I'd say that given cycling fitness, a bit of running and mental strength then you should be able to do 140km so long as the terrain isn't too steep or rough. It's the steep descent bits that really tyre my legs although that may be partly as I can't help myself taking off down them like they're the final bit of a short fell race.

I did a 50 mile trail race (round Rotherham) about 9 years ago, and the sum of my training was riding to work and back (25 mile round trip) and short, sub 1 hour, local runs and fell races. Felt pretty tired after, but was walking as normal by 2 days later. Took it pretty steady, just under 10 hours, and made sure to eat and drink.