Running

Now, what distances are fair? I can hardly run, but even a novice cyclist can move fairly quickly.
Help?!

I'd aim for a 6km run and a 30km ride.
If you go higher than this remember that running is more destructive. So maybe 10km run and 55km ride.

I did the wet test last night - turns out I have really high arches. Just a ball at the heel and another one at the forefoot. So I've ordered an appropriate pair of cheap trainers online. They'll miraculously make me loads fitter, right? Right?

The main difference between running and cycling is the destructive nature of running. Well selected shows reduces this and is vital.