Lucky you. I've wanted to do the Pilgrim half for ages.
Buster is a nice hill. If you can run up that I'm sure you can run up anything.
South Downs half isn't the toughest of halfs by a long way. Though I think being familiar with a route makes it easier. I don't think you'll have any problem with the Pilgrim half judging by where you train.
When I had a stress fracture I couldn't weight bare on that foot. Couldn't even stand. Not saying it isn't a stress fracture as mine came about during a race in which I struck a large rock underfoot rather hard. (Then kept on running, as you do).
If it is a stress fracture, continuing to run is a no no. You can walk on it though. You will need to get yourself to your GP and get an x-ray. Unfortunately stress fractures don't always show up too well on x-ray.
If it isn't a stress fracture it could be an injury to the tendons. This isn't good news. Any injury will take several weeks to heel. Doesn't really matter what it is. I was out 12 weeks with my stress fracture and a similar time for an inflamed tendon in my foot.
It goes by quickly and you'll soon get back into it. Running on an injury will only prolong or prevent recovery. Not sensible.