Two 6:1 stages makes for 36:1, not 12:1. Stage ratios are multiplied to get the overall reduction. That setup would get you a bit over 3mph!

Mod .7 sounds a bit small to me (roughly the same as what's in a drill gearbox), and I'd be even more cautious at just 5mm wide. I'd expect to use mod 1, 10mm wide, or similar.