I go out on trails with my dogs off leash all the time. I'm like martemchik
that I don't let my dog get too far ahead, but the distance changes with the hills/curves/trees. I need to be able to see them.
That said, only in areas where I won't see that many people/dogs. If it's really a busy area I just can't trust that many strange dogs/owners so the leash stays on. Since I'd have to keep them so close anyways just as soon use the leash.
AND I never do this without a reliable 'come' and knowing I can instantly get both my dogs back to me and off the trail so I can let the other dogs/people go past. Or get my dogs leashed up if needed for a bit.
Since I have ZERO control over the other dogs on the trail, I know I have to do all I can for MY dogs to train them to listen and come back to me. As well as know they are so well socialized and trained that no matter what dog comes at us they know that I will deal with the dog and they aren't allowed to. Plus I know they won't ever be the one to start a fight.
As far as running along and allowing some strange dog to nip at my heels, that's not going to ever happen whether I have my dogs with me or not. I don't let dogs chase me, specially little dogs. Either they are lost and I need to catch them to get them to the shelter. Or I turn around... make myself as big as I can with my sternest deepest I really mean it voice and I will move into their space to get them to move off.
Either way, it's never my dogs 'job' to fight another dog for me. Seen to many dog fights where both the winning and losing dogs have expensive and painful vet visits. Plus encouraging my dog to choose to fight EVER isn't in my training regime. Though I have to say that with my totally socialized dogs that know all their calming signals and ways to read the oncoming dog tend to calm down the situation even from behind me because they are calm, controlled and NOT adding to the chaos the oncoming dog may be bringing.
Takes 2 sides to fight and if I won't fight, and my dogs won't fight ... tends to take the wind out of the other dogs bluster.
But if the oncoming dog is in for the fight then my plans involve yelling, grabbing the 'new' dog by the hind legs to lift it up and toss it away (or spinning works too) and if I have to grab a branch and beat the crap out of it that's fine too.
This last scenario is as likely to happen in my yard/neighborhood/real world as out on a running trail though.