Hiking is amazing because it is free except for modest costs of things such as gas and sometimes parking. One great trail that is not too far of a drive from Fresno is the Mount Whitney Trail. This hiking trail is near Lone Pine. It is typically open from March to November.
The Mount Whitney Trail is predominantly used for hiking so you have minimal risk of hitting horse excretions like on other hiking trails. Additionally the hiking trail is amazing for endurance training as it is approximately 22 miles out and back. This is perfect if you are doing marathon training and don't want to completely vegetate on your day off.
When you go make sure to wear layers. At the bottom of the trail it can get extremely hot. However the temperature goes down as you go up and it is not uncommon to see snow and ice around the top around 13,000 feet of elevation.
If you are planning to go make sure to plan ahead. They use a lottery system to determine who can and who can not use the trail on any given day. There are days when it is so crowded that you can not go on the summit if you did not enter the lottery system.