I restarted cycling after a dozen or so years off the bike last June. 9 miles daily commute, mostly flat.
From about five months in I started going for actual bike rides once or twice a month, mostly in the 15-25 mile range. After the new year I started to pushed that distance up by about 5 miles each ride, with the rule being that I'd always try to get home thinking I should have been out longer.
At the point where I was at the 40 mile mark, I was persuaded by a certain CCE/YACF member and Audaxer who shall remain nameless(unless I say it was DaveC, because it was), to go on a 100 mile ride with him, three other experienced riders and one other semi-newbie. The first 80 miles were great fun, but I bonked horribly soon after that, sadly while far away from cake shops, and needed lots of towing to get me home. It also turned out that it was really a 120 miles ride. Ah well.
In hindsight I might have coped a lot better if I'd eaten more. And also if I hadn't been on a single-speed with flat bars. Sore wrists.
Since then I've taken an old but good-in-its-day MTB with the travel taken out the elastomer front suspension, 2" road slicks and bar-ends around the Highlands, mostly 50-60 milers, although they seem to have pretty big miles up there with all those braes. I never use the bar-ends normally, but normally that bike only ever tows my trailer around town on short runs. Found them invaluable when I was needing a change of hand position every so often.
TL;DR - I think once you've worked your way up to 40 miles in 5 miles jumps, then you're mostly worrying about food. MTBs don't need a huge amount of work to be rideable over decent distance.