You can choose many trails to get to the top....starting at the bottom in Clayton, Walnut Creek, or Danville. Or you can drive into the park and get a map and choose trails...there are gates in Walnut Creek, or Danville. The hiking trails can be long 13 miles in Clayton, or short and steep.... less than 5 miles....or you can shorten distances by driving into the park and picking a trail within. In any case....Mt. Diablo can be quite hot in the summer. Spring (better) and fall (brown and sometimes hot) aren't so bad, but the air quality may limit your views once to the top. I have been to the top hundreds of times. I am a cyclist and it is was my primary training route for a good climb/workout. I encourage people who want to hike for the 1st time to pick a nice clear winter day. It really is quite impressive in terms of the distance you can see. It is not overwhelming or spectacular like Yosemite or Lake Tahoe peaks, but, again, especially in winter when everything is green it is quite impressive! Summer and fall in the east bay will result in massive brown views in the valleys surrounding it, not my cup of tea! Always a good hike, some great mountain bike trails, or road cycling! Mt. Diablo also has some really nice campsites at various points along the way to the summit. Check at the gates for camping info.