From experience as long as you use decent quality cable you'll get 100Mb over a distance up to 100m. Hell, I knocked up a really bad patch cable a while ago made from old phone wire and gaffer tape. That worked at 100Mb over a distance of just under 10m.

About setting the routers on the same subnet, already done that. The wireless router isn't actually doing any routing, it's only acting as a switch. The modem does all the routing...

And about planning, I did a dry run this morning leaving excess at either end of the run. 15m should be OK. If it's not I've got plenty of spare patch leads lying around that I could use to extend the run if I need to. I just ordered a 5m lead and 2 joiners (the second one as a spare) to go together with the 10m that I already have. Hopefully they'll arrive before Wednesday.