I went trekking to a place over 5, 000 ft above sea level recently. i do not usually exercise n this is my 1st time. i did not get altitude sickness.y?

No: You would not get true "altitude sickness" but the elevation with intensify your lack of preparation of this more intense exercise thus decreasing your exercise potential and endurance.

Not surprising: At that altitude, true altitude sickness would not be as likely as at much higher altitudes. Also, people's proneness to altitude sickness does not depend on their usual exercise level. Things that can predict it are a history of having it before, doing vigorous execise before adjusting to the altitude, ascending too quickly, drinking alcohol, etc. Keep up your exercise!