I ride footpaths all the time. I'm not actually sure of the legality of it, to be honest. If there's a signed cross-country cycle trail, I'll usually do that instead, but most of the time there isn't. I've never had a problem with walkers, partly because I tend to be in places that are "isolated" to say the least, but a lot of it comes down to common courtesy - and that's not just slowing down for / stopping and talking to other users of the path, but also knowing when to get off and walk / carry (fairly common around here anyway for safety reasons) rather than churn the path into a mudbath.
The signed trails around here don't tend to get you anywhere; the prepared trails, which are almost all DH or at least hardcore enduro simply aren't for my bike (or me).