My 72 year old wife (Becky) has been on hydroxyurea (Hydrea) since summer of last year and has had no side effects at all. None! She started on 1x500mg capsule daily but when the platelets were not going down enough was put on 2x500mg 3 days per week then 1x500 on the other days. That helped and she is now down to 8 capsules per week.
As background, she is a very active person having competed in 15+ half ironman triathlons, run over 20 marathons and completed one full ironman.
The disease, (ET) and the medication has not slowed her down at all. Age has as she hasn't been competing for the last 10 years but again, it's age that has slowed her down a bit. We still have fun biking, running and swimming. Hopefully you will find the same thing though I know people are different and react to meds differently. Good luck,
Steve