Luke,
Congratulations on your excellent finish in the half-marathon! The general rule of thumb is that the body needs approximately one day of relatively easy recovery for every mile of a race. For a half-marathon, your recovery process should take about two weeks. During that time you can still run (or cross-train, etc.), but you want to avoid workouts that are longer or more intense than normal. Also, giving blood impacts people differently. I have heard of folks who are fine the next day, but I generally need a week or two before I feel like I'm back up to full strength. I really struggle to make it through my workouts for at least a few days following a donation. The lower leg pain is something you want to monitor closely. Maybe take a few days and do some low-impact cross-training to see if the pain goes away on its own. It would probably be a good idea to ice the area and maybe take an anti-inflammatory. If the pain continues you should probably see sports medicine doctor.
Good luck and happy running,
Jay