Due to having problems with my joints I either run off-road (round a field) or along a footpath. I have no idea how far I am running, though I go very slowly so I don't imagine it's very far. Want to eventually join a parkrun but don't want to 'run' before I am confident I can go the distance -- or at least get pretty close. Has anyone found a reliable way of tracking off-road distance that doesn't cost the earth?