i run marathons in km and for some reason find that easier as they pass quicker! i normally focus on 10km markers so 10km (quarter of the way there) 20km, (half way there), 30km (almost there) then i count down the last 10km to 40km and consider this my 'cool down' with a 2km sprint finish added on at the end!
do you want to do the challenge or not?? If you do then just get a plan and get on with it! you might find you enjoy it. Your fitness certainly wont improve if you continue just doing the same one run week in week out.