Start slow. Really slow. I am only about 175, and had sprinted in the course of playing sports over the last 10 or so years, but had not done any jogging/distance running since high school.
I started out too fast, got some “shin splints” or whatever and it really hampered my other activities.
Get some good shoes, I went to New balance.com and found a resource where your footprint kind of determines what kind of shoes you should get. This really does make a difference. I wear Nike Free’s or chucks in my day to day and sprint work, but for distance, getting a good pair of runners helped a lot.
Take care of tissue work and mobility in your ankles and hips. Check out anything by Mike Boyle or Cressey and you should find some good mobilization stuff. I start by rolling on a Lacrosse ball then ankle mobility, roll out hips and thighs with LAX/ and foam roller then hip mobility.
When I started over the 2nd time, I started with 1 minute jog:1 minute walk for 15 minutes, then just added a minute each time. 3-4x per week if you have the time. Once I got to 30 Minutes, I started going 2 minutes Jogging to 1 walking, and kept on adding to the jogging while eliminating walking until at 30 straight minutes.
You may feel like a pussy for the first little bit, but that’s better than being hurt. I’m going 5+ miles now pretty easily at about a 7:30/mi pace. Not super human, but pretty good. My lower legs feel way better and my sprint/sports performance has improved slightly. Calfs got a bit bigger too, without any direct calf work.
As far as losing strength: That’s why I started off really slow. it allowed my body to never get pushed too far and affect my lifting. My conditioning rose gradually and never impacted my strength work. I’m following 5/3/1 and still making PR’s every workout. I’m on my 4th cycle and still hitting 7-8 reps on my last set on 5-3-1 day, so my strength is still increasing. You just shouldn’t try to do too much too soon with running. Its just like the philosophy of 5/3/1, start off lower than you think you should and keep making progress for a long time. It may seem too easy, but taking 3 months to get to 30 minute jogs 3x per week isn’t that long of a time, and the slow buildup will allow you to get used to the increased volume.
To not lose size, I just started taking an extra shot of Olive oil at one then two meals per day, for an extra 300 then 600 calories or so, and I’m not losing at all. I’m not really trying to gain mass now, so I can’t say if endurance work and mass gain can be done together, but that doesn’t seem to be an issue for you now.
You’re a lot bigger than me, and I managed to mess myself up by doing too much too fast (seeing fat women running further/faster than me at the onset was a HUGE ego destroyer, but now I am at a much higher level, making more progress in 3 months than they have made in their whole lives is a huge ego booster).
So, Start slow, don’t do too much too fast, and take care to stretch and address tissue and mobility work.
Best of luck, let me know if you have any other questions.