as far as i know keeping your heart rate up is what makes cardio, well cardio, there is a target heart to follow, i believe it is 60-80% of peak performance, whereas 90% would be something you hit during interval training.
As far as what i do,it goes like this. I use the elliptical, so the speed depends on you. use that over the treadmill, because need really good shoes to prevent my shins for burning like crazy, it is said you get the same workout from either machine. but anyway, when i go i set a goal of at least 7 miles. which means i go about 7 miles an hour on the thing. of course its a bit easier because you don't have to deal with the imapct from actually running. today i did 7.5 miles in the 6.5 min (5 min for cool down), but i was at 7.16 when i actually hit an hour, so a little over 7 mph. I try to stick with what i decide. monday i did 8.03 miles, which wore me out! but it was fun.
ok next. Getting up to an hour. its all in your head! i said i could never do anymore than 30 min on a good day. then one day, i finished and got to thinking that i could probably go for an hour if i wanted to, so i tried it and it was fine. this was maybe a month ago. it felt like forever, but i pushed through it. my suggestion starting out is to cover the time and just listen to music. It helps a ton. Nowadays, its a breeze. i feel cheated when i can't do an hour. I also no longer need to cover the time or need my music. its just habit now.
so in summation, don't let your mind trick you into thinking you can't do it. I will say this though, there will be a point where you feel like you can't do much more, and its just in your head. If you push through it, your body will get a second wind of sorts and you can keep going. i have a friend who started coming with me the past week and he has already done an hour. i worked him up to it, but he got it done. he goes 5-6 miles an hour though, but everyone goes at their own pace. I have a ton of stamina from playing tennis and such, so that is why i go a steady 7-8 mph. Also on my strength training days, i do circuits so that i keep the heart rate beating while lifting, giving me additional cardio along with it.
hope that helps!
oh and don't worry about the girl thing. i am just glad i clarified in time!
oh one more thing, if you feel comfortable running at that speed where you are not about to apss out then i think you are doing fine! i think i can get about 3/4 of a mile at 6mph before my shins get upset.
And if you don't get under 300, no big deal. You know very well you'd get there the following week. 
