I echo everything above - BSc took me 10 years (7 years of studying) fitted around various 'career' courses, busy deployment times, children being born etc.
I'm now studying my MBA and so far so good.
The OU support is superb, they are flexible and the delivery technique means you can study anywhere (I've studied in the desert, in a tent, in 5-star hotels and down the back of pretty much every type of transport aircraft).
The OU is also very good at recognising your military 'quals', I don't know about recognition for all ranks (the learning education resettlement centre will) but - graduate of Cranwell = points; wear a brevet/wings = points; ISS/JOCC/ICSC etc = points. And we all know what points make....
I say do it - you have nothing to lose (Enhance Learning Credits will pay!)