I run every morning, sometimes to kilometer 0 and sometimes to kilometer 10. I've always felt safe. We stayed at a resort around kilometer 13.5 earlier this year while TTR was closed and, again, there were no problems. We've also taken a couple of trips to Walmart without incident. Other than that we really don't leave the resort.
We have hardly left resort as so good. But on few occasions we have yes total feel safe, even get on bus with locals absolutely lovely people
2 years ago we went to visit La Isla to shop: very nice and totally safe. On our way back we visited the flea market near Coco Bongo. Not that we felt unsafe but a bit awkward because of all the shop-owners trying to talk you into buying their goods.
Over the many trips to TTR we have left a few times. Walmart or to the shopping center. Last year we did the aquarium an afternoon and Hard Rock one night. It's all safe as safe as anywhere I've been in the states. Yes there was a few sketchy times but if you look for trouble it will find you. We may have been pushing the border of trouble, and we came really close to finding it...
I fell asleep on the bus ride back to Temptation after fishing one day and woke up not knowing where I was. But at no time was i in any fear or danger. And we normaly go to Walmart at some point in our trip.
Venturing down town to the many local shops, markets, bars, la isla shopping mall,walmart etc during daylight hours is no problem whatsoever, the buses are regular, cheap, and actually quite a lot of fun, you are travelling with the local population whom are generally always friendly and helpful, that said, our personal experience was that the vibe changes quite a lot after it becomes dark, many touts around the club area downtown etc, Myself and Margaret although never in any direct danger, that we were aware of, just started feeling slightly uncomfortable, and as such decided that whilst we had no reservations about travelling around on our own during the day, we would, in future only venture out at night whilst with organised groups from the resort, of which there are many, hope this helps.
I have been travelling to Cancun for 10/12 years and have ventured to the party zone, walmart via bus and even into the real city of Cancun for dinner. As always be vigilant and smart, aware of your surroundings and keep flashy jewelry in the safe at the hotel. A visit to the party zone / Frogs / CocoBongo is always a fun experience! If it's your first trip, plan it for a night. But as someone mentioned......it's hard to beat the party at TTR!!