Thanks tricialitt, Spoons and Carl for your comments.
Ran Regent's Park Winter Series 10km this morning as my weekend long run. DId first 4 miles at 9 min/mile but got caught up in the action and finished final 2 miles in 8 min/mile for a time of 52:55. Woops! Hopefully won't pay a price for that. Still, it was good to get experience of racing conditions.
Shin splint seems "OK" - there was a 30 second period this morning where it hurt a little during the run, but then it went back to being fine. So far after 9 runs into training I am yet to experience pain when running (that aside). Occasionally I feel a dull aching pain when in bed or whatever, but not too often -- it only hurts if I squeeze it. Continuing to ice/heat, foam roll, ibuprofen, stretch (I can now touch my toes! Hurrah) daily. My next training run is Tuesday so it'll be good to get 48+ hours recovery in now. I might do a treadmill incline session on Tuesday because that doesn't seem to aggrevate things at all (and hill work is good).
Carl, I fully agree that only I can decide if it's worth it. The marathon is April 21st (VLM). I know full well the right thing to do is to just rest and not run a marathon, but I am determined to do this. I am not stupid, though -- I am OK with pulling out and deferring my charity place for 2014 if I feel the injury reaches a point where it's a no go. My physio is a marathon runner and is doing all she can to help me navigate this. I've been doing a lot of core work and at least one swimming session per week.
Will be interesting to see how it feels after 9 mile LSR next weekend. That will be 1h 25m of running - furthest so far was today (55 min).