I had the exact same problem weeks before I ran the Manchester marathon. I saw a physio who gave me some stretches to do. I iced it loads and took ibroprofen to take swelling down but the one thing I would def recommend is a foam roller. There's loads of videos on youtube to show you how to do it. I managed to get through the marathon without any pain so it can be done if you put the time into getting it right. Good luck!
today is mostly being spent foam rolling, drinking water and worrying!! Hairdressers in a bit so hopefully that'll take my mind off things.. Although she always ends up covering my forehead in dye so that'll look good when I'm sweating tomorrow!! Going to paint my nails blue and yellow in tribute to Boston to because I didn't get round to getting any ribbons... Might do my toenails too- beats the black that's already there!!!