Brian - understandable that your time is limited, in which case you'll have to just to the most you can while balancing everything else, in which case I'd say three sessions a week.
I would honestly say doing all your runs at an easy pace is the best option. Too many people get swept up in the "speedwork is essential" mindset which while is true to a certain extent for less experienced runners it's not necessary. If you feel good one day it's fine to do a few middle miles at a quicker pace but doing it judiciously and in low volumes is recommended.
I wouldn't recommend HM training off of two runs a week but if that's all the time you can commit then so be it - nothing can be done about it. If you have the time I would say trying to add a third run each week but in this scenario missing a run is going to be quite a big deal (whereas for more experienced, frequent, higher mileage runners it is less of a problem).
It's good that you can run and you're not starting from scratch. Basically you'd need to increase the length of one run to ~ 110 mins over the training period you have and take the second from ~25 to ~40 mins to supplement it.
The problem is that there's such limited scope for runs in your situation. You're going to find the long run difficult because of a lack of other medium-distance runs to supplement it.
The half will be very difficult off of two runs a week. Football training just isn't a substitute (which you accept yourself).
Edit: In terms of targets, your endurance will be low enough that a 10k will give you a range but the pace drop-off between the two is such that it may not be a reliable indicator.
Edited: 16/01/2011 at 21:27