I have found that (I probably knew but had forgotten over many years) that one of my best motivators is to do be involved in an activity, be able to see how my friends are doing and at the very least try to work towards a little of a goal for myself. This has been Duolingo. Whenever I forget to play a day or two and I see how much someone passed me by, it clicks that like 10 minutes yesterday would've helped.
That being said, I don't have much in my life that I am able or do compare myself to other people- well, because that's just silly. Especially right now where I'm getting back into ballet and having to focus on doing much of my own training to be in the classes I'd like to join.
But, I need motivation to do cardio everyday. And not like, "Oh, do some cardio today." In any of the fine running/walking/zombie running apps can you add friends? By add friends, I mean queue you, friends because you're it :-P
I in no way want this to be some running/I'm fitter/skinnier/better than you are, because I am none of those things and am fine with that. I just feel like a small kick in the butt every couple of days from seeing a group of all of us walking (me)/running frequently might help.
ETA: If we're going for "accountability" I'd prefer it to be with you guise- I literally have no one else (short of husband, immediate family and one friend who doesn't work out) that really motivate me to do much more than pull small clumps of hair out.