Extreme Fun running

So, getting over(another) injury I am persuaded to do a charity fun run by Mrs S.
5k or 10k she says. Well obviously I plump for the longer distance -it's a fun run;how hard can it be?
Only local small hills but nowhere flat and Strava records an impressive 850ft of ascent on what the organisers billed as challenging! Not complaining - just felt sorry for the other poor souls who thought they might enjoy a Sunday morning runaround a stately pile in Worcestershire.
Of course, I only have one gear so legs feeling like #### today and probably set myself back a bit in recovery - doh!