<human>: **Back story**

I' ve just started running for a short while (< 5 months and on/off for first three). Given up smoking during this, and have no been getting into running more and the possibilities it  is opening up to me. I have just about gotten down to a 25 minute 5km, and yesterday managed to get under 60 minutes for my 10km (first run of that distance). 

Performed my first 5km charity run, and managed it in 25/26 minutes and was so proud to do so, along with the giving up smoking.

**Current issues**

Currently i'm a student and am lacking a large budget for food (£12 a week, ~ $19) and am able to eat meat/vegetables every day and have a fairly good diet. 

However, i am looking to progress onto trail running, adventure racing and the like. With my main goal doing all three spartan races during a season for NSPCC charity. 

Doing so will require me to have a strong core, and have now just started a core workout guide [(14 Day core)] to try and start getting prepared for harder challenges.

However, right now i'm very worried about lossing weight, with 400-800 calories burnt during every running event, i have spoken to people and have been recommended everything from protein shakes, creatine all the way to milk and lentils. But i am looking to both tone, become a better runner. Losing weight isn't an option for me. So any suggestions are more than welcome. If i have any misconceptions, or am asking an idiotic question please feel free to poke fun and help me improve.

Can you give me the gist of the text in a nutshell?
<bot>: How would you suggest i can maintain my weight during cardio, and then improve core/strength without spending lots of $$$ on food?