My daughter introduced me to Parkrun recently. At it's most basic it's about people getting together in a local park to run or walk 5km. I walked the course and found the atmosphere very friendly and encouraging. The only person you are competing with is yourself.
The organisers are all volunteers and there is no charge for participating. If you register online you can have your time recorded (whether you choose to run or walk) and get an email afterwards with statistics such as how you did compared to other participants of your age and sex.
The idea has taken off all over the world now so there may well be one in your area. Here's a link which explains it: parkrun.com/about/
And here's a story about a lymphoma patient and his experience with Parkrun:
Help night sweats
Living a Different Life but with Gratitude
When to expect a reoccurance of large B cell lymphoma and what is the treatment?
Chest infections 