Purple, it's always been a bit ambiguous but I've definitely read 5 miles in the current document somewhere.
I think it's previously been said that it should be in your local area and you should be able to walk or cycle to it.
The aim really is to stop large groups or people travelling long distances. My view is that you could drive a bit further than 5 miles , you might not get parked as a lot of carparks have tape up meaning they are meant to be closed. But if you can and it's really busy, you just choose somewhere else.
For lots of folks, staying local has meant it's harder to social distance. I'm sure something said at one point that your exercise should be in line with your fitness level. So a marathon runner for example shouldn't need to run round in circles round the park but that would be suitable for a casual jogger.
DP is a hillwalker (to a strenuous level) walking round in the flat doesn't really work for that so he has done his standard training quick up and down on our local hill, only a couple of miles away a couple of times but he'd normally do that every weekend if he wasn't going to do a proper hill. Local hill is full of mountain bikers though so it's not pleasant at the moment anyway.
He's been gutted at all this weather and not being able to get up a hill. He's late 50s so he says he only has a few really big ones in him now and they need good weather and an overnight stay. He's been trying to do one particular one for the last 5 years and never made it yet due to weather.