Definitely agree 5 mins is a good starting point.

I got some unexpected inspiration on my run tonight, it was dark, rainy and my first longer run in a month. As I went into my 6th km I was feeling very unfit (& regretting my bad eating over Christmas), I passed a gentleman running in the opposite direction who I know is over 80 - he smiled and waved. That kept me going the rest of my run. He doesn't run fast these days but he gets out there and keeps active - inspiring!