After a few months of inactivity I recently purchased a bike on Craigslist. I rode it around my neighborhood a few times, each trip further and not more than a half hour. Three days ago I drove to The Centennial Trail and rode for 1.5 hours. It was a hot day and I had water onboard which I drank frequently. Still, it was too ambitious and yesterday I had lightheadedness and toppled a few times showering and getting dressed. The walk in clinic ARNP said just to maintain hydration I needed a minimum of 2 - 3 liters a day; physical activity and increased temperature requires more.
A valuable lesson, hope this benefits someone.