I would say YES! because I think rest days are very important when you're training for a long distance race.
I hate following specific training plans. Sometimes I just don't feel like running and having my expected mileage written down makes me feel guilty about wanting to stay in bed or at home on my computer. Sometimes I don't feel up for a run (especially a long one) so I'll want to ride my bike instead or just do some easy Elliptical cardio. The master of rationalization that I am, I have come up with 3 reasons why cross-training (or resting) are great for running:
+ You'll strengthen your body by using other muscles, which will improve your overall training.
(I hear) this one especially applies to strength training. I don't do as much of this as I should, but I did have a brief love affair with weights last year. While I was working on my core, legs and arms, I found that my legs had more power and running felt easier. I hear that with a good core (I wouldn't know) your running form improves and this should prevent injuries by reducing impact force and bad foot landing (so overall technique).
This could also incorporate swimming. I know I find swimming difficult since I have tiny arms with little power to drag me through the water. However, the act of staying afloat develops your core and it's always nice to have toned arms :)
+Take a mental break from running
As much as I love the biology of running, I do understand that it's a very mental sport. You have to WANT to run and LIKE to run to get out there enough to improve. I always find that taking a break from running makes me miss it a little and enjoy it more. I also think cross-training gives me an opportunity to enjoy something else other than running. I would have never realised that past the intensity of swimming there's a sense of calm and quiet beneath the surface (haha) that I just love. (Sometimes being injured can turn into a positive thing!) By venturing a step away from running, I've gotten a peek into the world of other races - triathlons, for example - that I may want to try in the future.
So... don't feel bad if you don't feel like running today. Try and get out for a walk, bike or swim and enjoy the day instead of worrying about it or feeling guilty.