Sorry Imelda can't help you with the computer thingy and the Garmin 201 as I haven't tried it myself yet. Maybe you could ask someone else on the Gear forum?
Hi Dawn, BTing is base training, there is a very long thread about this on training, the first few pages of that thread explains it better than I ever could.
I take it this will be your first marathon Dawn, have you run other distances in races?
thanks everyone for your good wishes re:x-country race.