I think I've found something that has really helped and I want to share it with you in the form of a challenge.
Try this: Every day do push-ups, sit-ups, leg lifts, squats, lunges and pull-ups. Every....day.
How many?: Well this is the great part. Do as many as the day on the calendar. So on the 1st of the month do literally one of each (for lunges it's one of each leg). On the 20th do 20, on the 31st do 31. Easy right?
OK, so the end fo the month sucks but here is what I like and some rules for you rule bound types. This workout builds a habit with a very small time investment. At the beginning of the month its literally 2 minutes. At the end it's around 20 minutes. Everyone has that kind of time. You get some recovery time when each calendar month starts new.
Importantly do not do more than what is scheduled on the day! On the 1st do only one! I promise this is a smart tactic. At the end of the month if you can't do all 30 or whatever of the movement break them up throughout the day, it's not a problem. Do 10 in the morning, 10 at lunch and 10 more in the evening. My pull-up bar is in my garage, I often stop on my way to the garbage bin or recycling and do my pull-ups, it's not a big deal. There is no need to change into workout clothes. I've done these in my office at work, in my pyjamas at home, wherever.
If you can't do a single pull-up no worries, do assisted pull-ups with a band or do drops instead of ups. You can find modified versions (easier) of each of these simple workouts on YouTube. Do them as correctly as you can, form is important (I'm working on keeping my chin up for pull-ups). What I like is that this routine requires minimal equipment (you can do pull-ups anywhere, a playground, a door frame, be creative) and almost no time.
Fyi, leg lifts are done on your back with your legs straight. Put your hands on the ground and lift your legs until your ankles are pointed at the ceiling.
I've been doing this since September and I've noticed that my shoulders, upper back and chest have gotten significantly stronger. I've since added weight (a weighted ball) for sit-ups, squats and lunges. Just hold the ball close to your chest. I still can't do 30 pull-ups in a row, but at the beginning I could only do 3 or so and now I'm up to around 17. Most importantly for runners is that my posture has improved while running. I feel stronger and my hips/glutes feel more engaged. I've noticed it's easier to climb hills.
Now since doing this I've started a fairly regular gym routine at Planet Fitness, but nothing crazy. I'm still doing the pull-up challenge and so are three of my running friends. Give it a try, I hope it helps you find a cross-training routine like it did for me.