Hi Wyni I'm a Newbie in my 50's too. I also have done two of the c25k runs. I live in Australia so the weather is really starting to warm up so I decided to use the gym running machines. When I have had muscle pain in the past it's been sore legs when trying to walk after a hard work out.
There a lots of advice out there for sore muscles. Magnesium supplement is for muscle health. Having warm muscles is important, compression shorts help with muscle temperature and blood circulation. YouTube has loads of stretching/warming up routines to try.
Hope you have a more comfortable time on your next run