I don’t know what it’s like where you are, but the weather around here has been great for running being around 0-10C (around 40F) most days. I don’t like the summer heat and it’s nice to be able to really increase the mileage and speed again without fear of heatstroke. Some people thrive in the heat, and I gather you’re one of them - but I like it cool and raining.
After a long run I stretch (my body) during which time my penis returns to a more relaxed state again. Unfortunately I then soak my legs in an icewater bath for a while which, while it really helps all my joints stay nice and happy, is not so good for other things which can be found below the waist.
I usually follow that up with a hot shower, a few jelqs and a bit of light stretching (of the penis) returns things to a normal hang.
I personally wouldn’t want to run with any kind of weights down there, it just doesn’t seem like a good idea as I don’t like things swinging around to begin with. I generally prefer more supportive undergarments when running, especially in cold weather. If things are swinging free they’re unlikely to be protected from the elements, and around here that is a bad idea as temperatures will often reach -30 on winter mornings.
A bit of cold won’t interfere with your PE gains. It might even help, and it’s great for sperm production!
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Take your time, pick a simple routine you can stick to, and be consistent in your efforts. Patience is the key.