H.L.
I started with my boys around 8 weeks. I bought a stroller with 20 inch wheels - which provide for a smoother ride as well as less effort with pushing from you. I have a single and double Kool Stride stroller which I love. They have adjustable handles, fold flat and recline and have a 5 point harness. My boys are 3 (almost 4) and 2 (they are 17 months apart) and I have no complaints from either stroller.
Good luck with running!