I've been out twice today. First to walk the DCs about a mile and a half in total to a a playground. Not really much in it for me other than daylight. Then a run with another low risk SAHM which my watch happily confirms as exercise reaching a heart rate of 178bpm 
My main constraint is the combination of daylight and homeschooling.
In lockdown #1, while everyone was fretting about Michael Gove's mixed messages on exercise, I did lots of 1-2 hour runs, heading off to the quiet countryside where most people from my suburb would never think to explore. Social distancing was an irrelevance. If I'd bought in to staying very local and doing a suburban 5k, I'd have passed a hell of a lot more people!
Do whatever exercise you want to as long as it's reasonably safe (no Converse up mountains with your lunch in a carrier bag
) and avoid obviously busy places. Don't park your car in a nuisence way or leave litter etc.
I need more formal exercise in lockdown because I naturally stop moving. No school runs. Sitting on a chair for 5+ hours making sure the DCs learn. No popping out to catch a pokemon. No fitness classes. I can easily spend a whole day totting up less movement than it takes to get to 9:30 on a school day. Being short means that unless I want to exist on a restrictive 1300 calorie day diet, I need to be active to eat a respectable amount of food that is not tightly controlled. It's very easy for me to gain lockdown weight.
It suits the government to be vague anf let the curtain twitchers blame runners/ cyclists looking after their health. Really they would benefit from a fitter nation, but they're not that forward thinking.