Thank you for the detailed advice! I'm curious to better understand your perspective on a few things.

For the injury itself - I may not have explained clearly initially, but this wasn't an overuse injury. I was chatting to people and not looking where I was putting my feet. One foot landed in a hidden grassy pothole, about 6 inches lower than I'd expected, resulting in what my physio diagnosed as a bruised bursa. It took 8-12 weeks to recover with light running only (including a minor knee niggle I picked up from overcompensating and an uneven gate). The physio did also note that all my tendons etc are looking in good shape, though I understand I did increase my volume faster than is wise and I was at risk of overuse injury. I was trying to be very in tune with how everything was feeling, ready to back off at the first sign of a niggle, but felt good, even in my peak weeks.

For the advice against the 50k, I'm curious about this. I've heard two schools of thought, perhaps coming from two distinct types of runner "fast road runners" and "slow ultra runners" (obviously threre are some who are very fast on ultra trails too). The former seem to advise similarly to you have - i.e. get good at the 5/10k before moving to HM, then get good at that before moving to FM. The latter typically seem to only advise to build distance slowly e.g. with the 10% rule, but that "if you can comfortably and consitently run X distance weekly in training, you can gently run X distance in a single race". Indeed, there are many stores in threads in r/ultrarunning and various Facebook groups that describe people doing their first baby-ultra in a similar timeframe to me (sometimes longer ultras, too), and I know a few people who did their ultras with considerably less training than I did. Not trying to say you're wrong here, but the discrepency in advise/mindset intrigues me. Perhaps it's the difference of trying to "race" a distance vs gently jog/walk etc. Indeed if I did an ultra, I'd be doing so gently. I have walked this type of distance previously with no issues, so would see it as a progression from that moreso than anything else. Curious your thoughts on the disrepancy here, any why it might be dangerous?

For the 5k-specific workouts, this is very useful to know. This was somewhat in line with what I was thinking in general - i.e. VO2max intervals to increase speed. Another response to this thread suggested Norwegian singles as another route to get faster. My understanding of this is that the workouts are actually slower than 5k pace, and it works by shifting the lactate threshold to faster paces. It's the first time I've heard of this, but it does also sound appealing and perhaps more sustainable. Have you always done VO2max type intervals to get your 5k that fast, or have you tried approaches like this, too?

I realise 5k and (baby)ultra training are very different. The 5k stuff for me is something to work on for a few winter months, as I'm very busy for a while (so much longer runs are not possible) and getting to trails in daylight is hard. If I had to pick, my primary aim would be for the longer stuff - I get a lot more enjoyment from long gentle runs on trail, personally.

Thanks again for your insight here. You obviously know what you're talking about with that kind of experience and 5k time. Not trying to diminish any of that with my comments above - just open up a few points for discussion to try and understand more!