I am the previously- mentioned "typical" ultra runner, becuase I used to be able to just break 4hrs for a marathon, and I can't any more- however I am usually in the back 10% of the field, so presumably the other 90% don't fit lardarses criteria!
I think that road marathons are fine (I've done a handful), but an Ultra is a much more sociable, relaxed day out ( perhaps that's why I'm near the back). The ultra community (in scotland anyway) is a really friendly bunch.
The advice from a variety of folk on FB, including a race director, and a few fellow slowish guys that know my pace, is that I would probably be fine for either, but the stress of stepping up significantly from the 50-ish miles, and worrying about cut0offs, makes me think I'll plump for the 24 hr race, to just prove to myself that I can keep moving for that length of time, then maybe the 71 miler next year. I might change my mind again, but I think that's a more safe/ cautious way to go.
I don't know that I would ever do a 100 miler, but the WHW race (95miles) sounds so awesome, that I have to admit to being tempted. Probably only something I'd try if both the 24 hour race, and the 71 mile race were OK. ...............trouble is I will be 54 by then- and there's no guarantee of getting in on the ballot, but hey- I could get hit by a bus tomorrow, so no point planning too far ahead.
Daniels is a fantastic text book of running. It will take you a wee while to read! .FIRST is not much fun, as all run are done too fast for comfort (I've tried it). Better to go for an 80:20 easy: hard type plan, I've been using P&D, but converting the miles to km's - their lowest volume plan is "up to 55m perweek"- so that converts to about 30 miles, and is fine for me!
You are just a touoch faster than me over 10k/ HM, and I managed 3:56 for a marathon at my peak - now slowing down due to age and ultra running
I'm sure that is just bonkers, and some one more sensible will be along with better advice soon, but hey , whatever.
Having said that..........a strap that lets you carry on running, and getting stronger might help! - I used a CHOPAT strap for the first 3 or 4 years, and now don't use anything, unless I use KT tape - try looking it up on Youtube. First you need to figure out whats wrong, and see if there's anything to do about it (eg I was wearing the wrong sort of shoes), but strapping or a support might let you carry on if you want to.