While being in good shape may help with your performance at altitude, it is no guarantee against AMS (acute mountain sickness/altitude sickness). It is more a matter of your body's own response to altitude. It can manifest itself in many ways including headache, feeling tired, nauseous, dizzy, Cheyne-Stokes breathing while sleeping, as well as the more dangerous HAPE and HACE.
And it's not just a problem for people living at sea level. Even if you live at 6000' and rapidly go to 10,000' yo may experience symptoms.
The ONLY way to reduce these symptoms in people prone to AMS is to acclimate slowly. If you know you have had issues in the past, you should consult your doc for a prescription for acetazolamide (diamox).
Some good info can be found at www.princeton.edu/~oa/safety/altitude.html
Don't worry about oxygen tanks, if you get that sick you must descend in altitude quickly and get to a hosp.
That said, the chances of that happening are very very remote. Most people (like me) just have nagging symptoms: headaches, heart pounding, can't sleep.
Here are the basics:
1)give your body time to adjust. The times I have the most trouble is when I fly up and go to high altitude in the same day. You should always try to go up at least a day early and stay at moderate altitude for at least the first day. This year we went to Durango and stayed at 6500 ft for 24 hrs before heading to the high country.
2) Avoid alcohol and caffeine until you adjust. They increase your chances of having symtoms.
3) Drink water like a fish. Then drink some more.
4) Take antacids (rolaids/tums) at 2 to 3 tablets every 6 hours.
5) I take a nutrition supplement called Altitude adjustment from Sun Countries inc. I start it the day before I leave and take them for about 3 days after I get there. Helps me a bunch!
6) If you are that worried, you can get your doctor to prescribe Diamox (acetezolamide) to prevent symtoms. Does have some side effects: makes you pee and makes any carbonated beverage taste NASTY. I used to take this every time, but don't have to anymore since I started taking the altitude adjustment
I never have severe symtoms, but they are annoying enough to make me not enjoy the first few days I am there. Doing the above has made the transition comofortable. However, the air IS thin, and that will take a few days to adjust too no matter what!
Prepare, then don't worry, just go and have fun. For years I did the above and did OK even though I lived in Corpus Christi, TX (sealevel)
You shouldn't have that much trouble at 7000 feet. I live in Austin and when I run in Denver (mile high city) I notice only a slight decrease in performance.
Many runners compete in the Pikes Peak marathon every year which starts at 6300 feet and goes to 14000+. At least 1/2 are from sea level. Everybody is different but I was able to complete Pikes Peak Ascent several times without altitude sickness or acclimizing - we would arrive a 1-2 days early but experts say it takes 2 weeks to acclimatize. The better shape you are in...the less you'll suffer. Go here for more info.