The San Joaquin River Parkway and Conservation Trust have several free and low cost options for hiking and more. These are near Fresno with drives under an hour which make them great for a cheap fitness trip.
The first is Sycamore Island Ranch. It is located in Madera near the Valley Children's Hospital along the San Joaquin River. It is open from dawn to dust and the cost per vehicle is approximately $9 for parking. There are hiking trails on the 350 acre reserve. In addition to hiking one can go fishing and picniking. There is a bait shop and food available for purchase onsite.
The second is Woodward park. This is off of Highway 41 at the corner of Friant and Audubon. It is also open from dawn to dusk and the parking free is $5 per vehicle. There is amazing hiking options and also playgrounds and picnic sites.
The third is Jensen River Ranch. This is a hiking area that is free with no parking charge as of June 1, 2014 and is 167 acres of land.
Next is Lost Lake Park. This is open dawn to dusk and has hiking on its 305 acres in addition to self guiding nature trails for walking.
Next hiking option is Fort Washington Beach. This park is amazing for hiking and is privately operated at the end of Lanes Road. This hiking area has river access.
Hiking is also available at the Wildwood Native Park. You can reach this park by taking highway 41 north and exiting at Rio Mesa. You then turn right at the frontage road and left on the access road towards Cobb's Tree Farm. The hiking here is out of this world.