Why can't I gain weight?!

Run less. Eat more. Drink more alcohol.

Eat things like:

peanut butter

cheese

avocado

good cuts of meat

add olive oil to your food/salads

i.e. calorie dense foods.

"I run long distance why don't i gain weight?"

Try lifting, actual lifting. Not 3x20 pushups after your 8 mile run.

The whole problem is I want to gain weight to get a lot faster, since it 100% would allow me to do that. I don't want to sacrifice running training just for the superficiality of looking slightly athletic. It seems like the only option I can think of is waiting until summer and then I will be free to do whatever training I want. I wan't some more recommended foods, because frankly "cheese and nuts" are not enough to change me from being very underweight.

They probably are, actually.

Your shorter times are not all that good (68 is a pretty slow 400, more like 400 repeats).

I was small and thin going in to high school and grew a bunch but was still probably 5'10/125 for XC my senior year. I was pretty strong 100 situps, 17 pull-ups, top couple on school fitness tests despite being maybe the 20th best on the XC team. I was strong enough to be fast by comparison (56-59 for 440s).

I got suddenly stronger the last couple of weeks of XC my senior year and ran much better. When I started running a lot in grad school I was a much better runner (sub-3hour in my first road race), and had finally gained weight to 155 or so (with the help of basic training in the Army). I lost weight slowly but systematically over the next eight years down to 137 (had switch to racing on the bike mainly due to foot injury). I stayed that weight then moved up some as I started running less.

Then my system switched back again and I have been steadily dropping weight for for 15 years despite trying to keep my weight up for the last 7-10 years of that. I find that I can only eat so much before I am full and if I stuff myself it does not seem to help, in a manner similar to your description.

You should probably consult a sports nutritionist, and start by reading on the Web. Maybe you and I have 'overly quick' digestive systems and are not getting the full caloric load from our food.

Now I can run (im ballparking) around 63 for 400, still bad but its better than it was. I started my indoor season late and couldn't run for a month because I had pneumonia. My first week back I ran a 2:23 so I am much faster than I was as a freshman. Im growing so much now that I don't even know if I should care about my weight. I think I'm just going to use my shitty set of weights and do the basics 1-2x per week. If my weight doesn't change it doesn't even matter. I'm going to up my mileage and my strength training, and try to just eat as much as possible. Its annoying that when people look at me they see me as a weakling, but there are greater tragedies in the world. I completely destroy everyone with my kick regardless of their sprint times anyway. I guess the only reason I want to gain weight is because I'm tired of everyone making a fuss over it, and thinking i'm not athletic. I can't believe I was so ignorant to think I needed weight to run fast in distance. Farah was a 100 lb "weakling" when he first came to Salazar and he was a better runner than almost the entire world.

I'm 5'8'', 122 pounds, 24 years old. I was in your boat in high school, but since I began incorporating real weight lifting into my routine, I have filled out a lot. Despite still only being 122 pounds, I look like I weight ~135, thanks to the lifting and increased caloric intake. You may have the body type that will never allow you to get 'big' but you can at least fill out your frame by putting on some muscle. It will help your sprint time as well as your running. For reference, I was about 115lbs in high school but could still clock a 56 400m. In college I shot up to 120lbs once I started lifting a lot more and dropped my 400 to 48. I also dropped my half marathon from 1:27 to 1:22 that same year, despite 'bulking up'.