I had a similar experience where I was dong my road running in 4th pair of Asics 2120's (which was fine) and off road in my Inov8's (which were also great for 6 months previous), but I increased my distance off road from 12 to 14 then 15 over subsequent weeks as well as reducing my stride length & increasing my cadence (and not landing on heel) in a bid to reduce the chance of shin splints coming back. My calf just gave up in a sudden pull/strain which I ended up on Physio for 5 weeks for; he (the physio) reckons that it was the stride length/cadence change that put the extra strain on my calf not the trainers or distance increase.
I've now started almost from scratch again, slowly building up my distance with the new technique and do other leg exercises too. All ok and now 10 weeks into the program. Not sure if this helps or not.
I use a 'Trixie Hands Free Lead', which you can google and look at. Its ok - basically a thick elastic belt for you and a lead (approx 2m adjustable) with a small section of stretchy lead. Also come with 2 pouches (for treats, mobile or empty poo-bags (not really for the full ones!)). I did look at the CaniX leads/belts, but the costs were way to high for starting out in running with my dog. - maybe in the future.... The Trixie cost me less than £20, but you should really use a chest harness on your dog rather than its neck collar.