Paleo and Primal favors low-level cardio (ie - walk a lot) and brief high-intensity sessions (ie - sprinting) over sustained steady-state cardio, because that type of sustained cardio causes too much stress on the body, especially over long periods of time. It takes a toll on the immune system and has a high injury rate. Long-distance running is also highly catabolic (muscle-eating), and losing muscle means less functional strength for other tasks and a (slightly) lower metabolism. These things run counter to the goals of Paleo/Primal, so they're generally advised against, at least under the pretense of "getting healthier."

That said, I know PrimalRunner was doing long-distance running for a while (don't know if she still is, since she's doing a bulk phase), so she might be able to offer some specific advice. There are also a number of athletes that follow a Paleo or Paleo-esque ketogenic diet. Some will "carb up" beforehand with Paleo foods (namely sweet potatoes), while others won't, and their in-race fuel is often things like cheese or other high-fat items. As far as I know, most of them train in keto, even if they carb up for races. One big thing I noticed is that running while in ketosis (and for some, probably even at lower carb non-keto levels), is that you really have to make sure you get enough water. Lower carbs means lower glycogen and less water stored within the body, so you have to get hydration from somewhere.

Handyrunner- I don't know why I didn't think of this before... Larabars! They are great. Quick source of carbs, and they are made of dates and nuts, nothing more. They will be what I carry on the bike from now on (when it's warm enough to start riding).

My husband and I love them- I don't know why I didn't think of using them in place of the shot bloks.

I have seen big improvements in my immune system since I started cycling and running. I have far fewer colds and minor illnesses. I think one within the past two years. From what I've read, you can have a slight decrease in immune function in the hours immediately following a long distance run (or ride), but long term it boosts immunity. I guess it depends on whether you read runner's world or Paleo 101

Ironically, even Runner's World admits/acknowledges the strain long distance running has on the immune system and notes the difference between 5Ks and marathons - http://www.runnersworld.com/health/immune-it-all It even mentions to scale back your training runs and use HIIT, instead!

My running probably doesn't qualify as "endurance" but my cycling would. I log the most miles on the bike during the summer months. There are less colds/flus going around that time of year... maybe that's why I haven't seen any immune issues. My running, which I do during the winter, would be considered moderate. I guess it's all a matter of perspective.

Regardless, the running/cycling is working for me, so I'm sticking to it. I think exercise has to be about what you enjoy.
Many endurance athletes have made this work for them, and there are resources for triathletes, marathoners, and cyclists. If pro endurance athletes can do it with the crazy amount of training they do, then surely I can make it work. I know that strict paleo followers won't agree because the strict program encourages a completely different type of exercise. That's fine, but I think this can be adapted and work for those of us into endurance sports. I think even a modified version would be a healthy switch for anyone.