I also used to do this and I got very fit doing it. My commute was shorter, so at first I did 4/10-12 (doing a longer route in the evening - I was a 100mpw runner), but I actually found later on I ran better off more 7/7-8 type doubles on my easy days.
I ran in+out on my workout days too, I just took a detour through a park and did my workout there - it sometimes meant a 5-6mi warmup and 4mi cooldown but once I'd built up to it that was fine and it was still worth it in time saved.
I used to take the bus in monday morning, leave clothes for the week and anything else I might need in work, run every other time and take the bus home friday evening with my washing etc (so 2 days where I only had to run once, but I sometimes ran twice anyway).
Showers in work are great if you have them, but if you're discreet and not a big sweater you can also wash up pretty well in the bathroom with soap, a cloth and a towel. I was lucky in that any work I needed to bring back and forth could be put on a usb key, so all I had to carry was a pack with keys, phone, usb etc.
Do give yourself some time to build up to this if you don't already
a) run doubles
b) run on the roads a lot.
This is a great idea! I've massive respect for people who can run big mileage and also commute to work, but I'd never have been able to do it.