Hi!
Congratulations on deciding to move forward with this decision; I hope it works out well for you. I will soon be marking 2 years since my kidney donation in November.
Before I had all of my pre-donor screenings, I was nervous too--but this was more because I was really hoping to be an eligible donor. But it made me feel better to know that the eligibility factors were out of my control. I had already made my choice, and if it wasn't in the cards for me to be the donor, then so be it.
The actual tests weren't too stressful. I took a day off from work to do the 24 hour urine screening and tried to stay hydrated. Other tests included the blood draw (fine, as long as you're ok with needles), EKG, CT scan. I had to have x-rays because I have pins in my spine and my surgeons wanted to be able to see what that would look like and how it might affect my body positioning during the surgery.
I agree with @audrey12...the CT scan was a little disconcerting, especially if you don't like small spaces because you're in that little tunnel thing while they scan you. I later had to do a CT angiogram, where they inject a serum and then observe how it travels through the kidneys and urinary tract. This just involves lying down and waiting for the observation to be done and takes about an hour.
Like you, I'd generally call myself "fit," or at least I try to be--I try to eat whole foods and especially eat more fruits and vegetables and exercise every day. I was 27 when I donated, and besides the challenges of recovery I feel I have bounced back.
Good luck!