I have whippets- I race them and hike the plains of wyoming with them hunting jackrabbits. To prepare for a wyoming trip, I like to get about 3 miles several times a week alongside the bike. We run with salukis in wyoming, and I don't know if it's possible to tire one of those out. Those are the ultimate distance machine, if you want speed as well as distance. I have a springer (bike attachment, not spaniel!), and have roadworked as many as three at a time- one on one side and two on the other. That's kind of hairy, though, so prefer just one. I just ordered a walkydog because the springer is very bulky and I keep bumping it with my foot because of the way it fits on my folding bike. I only roadwork dogs with the bike on the weekends, so far, but really want to ride every day. I pick up my 4 year old at daycare on the way home, and there just happens to be a great park right on the way, so it looks like we'll be detouring there with bike and trailer most days.
I went to the bike shop and looked through all the saddles- they didn't have many of the ones that were mentioned here. Many didn't look all that different from my velo, but I kept coming back to one- a serfas women's "dual density" saddle that wasn't as wide as some, but definitely bigger than the one I've got not- not as long, really, but wider. The type of riding I do probably leans more towards comfort bike riding, but if this works out to be too wide I can take it back. I'll try it out today hopefully, I can't wait- I was actually dreading trying to ride the velo again today. I'd hoped to keep increasing my distances but that was looking like it was definitely going to limit me.