RunCheffyRun, I think that if the plan gives you a option to cross train that it is intended to be a recovery day, so you don't want to do a tempo run on that day. My suggestion would be to build up to 22-24 miles a week before thinking about tempo runs. In the meantime do a few fartleks, run tempo pace for 30-45 seconds 3 or 4 times during a run once a week. It's too soon to be concerned about speed. Trust the plan or find another because too much too soon will probably get you injuried.

RunCheffyRun, I think that if the plan gives you a option to cross train that it is intended to be a recovery day, so you don't want to do a tempo run on that day. My suggestion would be to build up to 22-24 miles a week before thinking about tempo runs. In the meantime do a few fartleks, run tempo pace for 30-45 seconds 3 or 4 times during a run once a week. It's too soon to be concerned about speed. Trust the plan or find another because too much too soon will probably get you injuried.

Double ditto, and I'll add once you get up to your 22-24 miles, run 2-3 weeks at that mileage first before adding the tempo runs. Let your body acclimate to the mileage. Also don't forget warm up and cool down when doing the tempos (10-15 minutes easy / recovery pace).