@REI-LizH I have a 3 year old black field lab, Charlie, who loves running with me on rides.  My tips are:  Bring extra water and a collapsible bowl, like Ruffwear Trail Runner.  Bring nutrition if your going over 15 miles (I share my granola bars).  Carry a hands-free leash just in case, I like the Iron Doggy Runner's Choice.  I don't recommend boots because they're finicky and often twist or fall off, but I'm in Michigan and the trails are pretty soft - we just warmed up to long rides and his paws have grown tough.  I also got a cooling vest that we never use, instead we have lakes and I'll stop so he can jump in if it's hot.  Probably everyone has a different opinion, but I let Charlie run in front of me about 15 yards and he doesn't get in my way.  I get a better sense of how tired/hot he is by watching him (and it's really fun to watch him run).  He's pretty good at slowing down for me when I call, so we can cross roads together.  Finally, I up his food amount afterwards and the following day.  I'm guessing he burns crazy calories on a 20 mile bike ride.  Lastly, if you don't already have a dog there's a huge range of capabilities from breed to breed, and even within the breed.  I've had English Labs and Field Labs, my English Lab was an awesome dog, but he couldn't hang with the speed and distance of mountain bike rides.  Lean dogs will fare much better than stocky dogs.  We have an awesome time together and really bond - I wish you luck!