incognito - way to go! You are getting off to a good start.
So if i understand you, your twice weekly workout is 30 minutes of weight lifting, sandwiched by two sets of 8 sprints. IMHO, this is too much for a single workout. Personally, I either lift weights intensely or sprint, but not both on the same day. Obviously everyone is different, and if you are feeling great doing this perhaps it's ok. But if you find yourself wearing down -- evidenced by Friday's workout being harder than Tuesday's workout was... then you are over-training and need to give yourself either more recovery time between intense workouts, or scale each workout back a little.
Best of luck.
p.s. - A stairmaster machine is also good for No-Impact sprint work when you are ready for more intensity than the elliptical can offer.
42 yo female; 5'8"
Oct 2009: 205 lbs
Dec 2010: 167 lbs
Current weight: 158 lbs (first time under 160 in 17 years!!!)
Goal weight: 145 lbs