Always enjoyed running in my younger yrs and had no choice when I was in the forces but not done so much of late but would it help me get stronger on the MTB if I did a lot more running for endurance wise ?
Ard Rock is fast approaching and very little bike time for 5 weeks due to horrendous man flu so struggling with fitness at the mo 🙁
Not specific to cycling. At best it'll strengthen other areas of cycle muscles, so when your cycle muscles are tired they have other capacity. I think it was Nick Craig who said any training time slot of approx 30mins he went running, otherwise bike. Somethings always better than nowt.
Not IME. When my mate (who was a strong runner) was first getting into MTB, he had very little strength/stamina on the bike (although more than someone who did no exercise, obviously!)
Thanks Gents
It won't do any harm anyway I suppose plus I need to loose some weight anyway so not a bad thing
when i run, i take a heart-rate monitor. I concentrate on specific intensities, it's hard. It's training.
when i'm on my bike, i'm having too much fun to think about my heartrate.
consequently, i'm at my fittest when i've been sticking to my run-regime.
yes, i could ride my bike to a training regime, but no. ****-off.
Sprinting intervals might help.
Or singlespeeding - which is a bunch of intervals 🙂 And can certainly help make you strong if you push yourself instead of pushing the bike 🙂
I've found 3 weeks from start is enough to see a reasonable difference in fitness/strength with a decent plan and some motivation
Edit: Mind you it also took me 3 weeks to get used to the idea of singlespeeding 
I recently started running again after a few years off. I'm relatively fit on the bike and I'm amazed how hard I'm finding the running. I'm only doing short distances to build my knees up but even that's killing me.
I found I lost a bit of weight when I started running last year which I think helped when on the bike
My legs seem to be dead for a few days after rides at the moment, not sure why but the thought of running as well seems impossible atm.
Wouldn't mind running on the trails so much, but running on the pavement is the most boring thing anyone can do, ever.
I'd focus on intervals and hills when running, this should help a bit with power on the bike. I'd agree with most others that just going for a jog won't do an awful lot for you. However one good thing about running is that you can fit a pretty good workout into a short time compared to (most) cycling.
I've started following up my shorter easier runs and bike rides with some circuits of mostly bodyweight exercise, squats/press-ups/crunches etc. I hope that will do something for overall core strength. Early days but it does seem to be having some effect.
Trained almost exclusively for a 16 mile cross country 'race' over winter this year for the end of March. Nothing scientific, just lots of running, whenever I had a slot I ran.
Was hopeless on the bike come Spring and time free to ride again. It really helped with weight loss and cardio, but my legs went after a couple of hours riding. So I think it's a good cross train for a session or two a week, but for endurance you need time on the bike. Maybe intervals / hill repeats if time pressed.