Got to ask how you chose your trainers? With gait analysis? The wrong trainers can lead to shin splints too.
Normally no need for 2 weeks after the half. More like 2 days. But if the shin splints are bad - you need to be resting for maybe several days at least, then ease back into it. Make sure those trainers are ok... and ideally, go to the physio. Good luck with it.
Richard... I don't know what you mean by saying your plan says 3.11. Do you mean that's your target time? If so, I trust you weren't trying to do the 28K at 3.11 yesterday (unless part of a reputable plan).
If the plan says 3.11, and you're thinking 3:30 would be ambitious... and thinking sub-4 is realistic... you really need to narrow down your target time pretty soon, because that's a massive spread, and you can't train well for all of those possibilities.