At first glance you start with perhaps the hardest session (1km fast 2 mins recovery) and then move on to sessions that soem would say were easier (ie 100m with 90 seconds rest!)
Not sure as well if you have thought about doing some shuttles to get the team work going. One of a pair does a loop of a circuit and the other rests while they are doing it etc then rests while his partner does it as more "fun" for a group,
My understanding is that it rarely sells out quickly and in past has been pretty easy to get a place for. No real need to buy through a sports tour company unless you want the security of staying in a group with guide etc.
If you only can do two sessions a week I think its a luxury to jog round a 10km in 45 ninutes (if your aim is sub 40) every week. I would probably drop that session to every other week or every third week and focus on two quality sessions a week. Would focus on intervals and tempo runs with the odd hill session thrown in. You should be able to get away with two hard quality sessions as you have so much down time.
Maybe do some longer intervals (say half mile one week, mile the week after) as well
Just treat Portsmouth as a fun LSR. Target Stevenage, have a week or two of relative rest and then just do a couple of steady mileage weeks. You wont lose much fitness. Think doing multiple marathons is as much mental as physical - people think they have to race them all hard and train as if each one is a stand alone event.