I love living near a city mtb park (especially since my county isn't shutting it down). I take a fixed gear down ~1.5m to the trailhead and loop the crosscountry looop once or twice and ride home. I get a great workout without commiting as much time. These trails have gotten so much use over the years that roots are worn down and most of what was challenging has erroded away, but it's still away from traffic, has up-downs-lefts-rights and gets me out there.