1. Try telling yourself that if you train for a marathon now, you will likely injure yourself and be sidelined for months, or even wind up quitting.
The most common time to injure yourself running is during the first 8-10 weeks. Why? Because beginners do too much, too soon. This is the beauty of the C2K - it forces you to go slow and work your way up. The 8-12 week mark is the most vulnerable time for a new runner, because your heart and lungs adapt quicker than your ligaments, so you feel like you can run longer and you do - and then you wind up injured.
Lots and lots of people take up running, get injured, and quit. You took up running so you are already doing better than the vast majority of couch potatoes. Your next step isn't to run a marathon (or even a 5K). Your next step is to get through the first 3-4 months without quitting because you got injured.
2. Remind yourself that you are doing this the right way.
Experienced runners will tell you DO NOT train for a marathon until you have a solid year of running behind your belt. I don't know about your friend, but if you want the respect of experienced, seasoned runners, you will not worry about a marathon for awhile. There will be plenty of time for that later.
In fact, if you complete your first 5K, I would recommend a 10K as your next goal. NOT a marathon. Do that, then try a 10-miler. Then a half. Then a marathon. Think in terms of years, not weeks or months.
3. Remind yourself that a 5K *is* a big deal!
Shorter distances are a completely different sort of race than longer races. It's speed vs distance. <insert complicated description of how the two types of races train different parts of your system). There are veteran runners who never run a marathon and only stick to 5K's, because that's what they are best at. There are veteran runners who only run long distances because that's what they are best at. I personally prefer longer distances myself. I do not like 5Ks. I still run them once in awhile, but I don't enjoy them.
4. Tell yourself "I'm doing better than Shiny."
I am running 30 minutes, 3x's a week, but it's taking me that long to complete a distance it used to only take me 20 minutes.
Also I can guarantee you are faster than I am! I am very slow. No matter how much I train, how much I run, I will never be more than a back-of-pack runner. But that's okay. I'm not doing this to win races. I'm in it for the mental and physical health benefits, weight control, fresh air, music on my ipod, and the exhilaration. And you don't need to run any races to achieve those benefits.