This thread gave me inspiration, so yesterday I went to Greaves and bought a pair of New Balance trainers. They're absolutely huge because of my broad feet, but have good padding.
Decided to do my first run through Linn Park today. Quite an interesting course. Very hilly at some points, and lots of stairs. Also saw some guys that used to go to my school, aged 14-15, drinking mad dog on the steps. I felt such pride.
I'm glad I did it. I play better at 5s when I'm at peak fitness, so hopefully I'll go back to being a complete monster at 5s again.