Started running again in January after a good few years layoff, entered the Janathon (google it) and got on alright, 155 miles over the month. Stuck with it steadily through the year and into the Juneathon (warmer brother of Janathon) and clocked 200 over that. And now I’m just on my way back from a beautiful but brutal 23 hrs on the Grand Raid des Pyrenees ultramarathon! (50 miles, 5000m total ascent)
Biggest changer for me was getting a decent gait analysis when I bought shoes – the Salomon XAs I’d run in in the past were ripping my feet apart, blistered really badly in my arch. Accelerate in Sheffield were really thorough with me and helped me sort out what was going wrong where (hypermobile gait, pronation beyond the limit of the motion control in the Salomons) and have sorted me out with a couple of pairs of shoes, Saucony trail shoes to start with, then Mizuno Wave Harriers to deal with the Ultra better.
Biggest tips I’d offer are to get a decent analysis, try and clock mileage off-road (easier on the knees , less boring, and helps build stability and propriaception into the legs), and just enjoy it!