Climb to 7500 ft or so to cross mountains eastbound, pick your route to match the climb gradient of your plane, the route may need to vary a little from a direct routing. Good climbing planes departing from westerly airports can easily go direct. A C150 from Gillespie maybe not.
For a nice leisurely arrival and descent aim for Borrego Sky Ranch (private airport, see chart) and turn inbound to make a long 45 to Borrego Valley (L08) right traffic Runway 8, which is quite often the active runway. Borrego Valley Airport (L08) is visible from there as being just south of the solar panel array - which is hard to miss. Make your pattern outside the elementary school just to the west of the runway (shown on Google Maps) - the Principal gets uptight when planes fly directly overhead.
Lots of traffic will be found around El Capitan reservoir, sometimes including commercial aerobatic planes from Gillespie going up and down fast (vertical between 3000 and 8000 ft), but returning from Borrego that way does facilitate the infamous ‘Dudley’s Arrival’ from Santa Ysabel, following the San Diego River valley - it takes you where you need to go and it’s the old timers scud running route
(Dudley’s is a well known bakery in Santa Ysabel)
Watch for sailplanes from Warner Springs, you’re in their soaring area on your return if limited climb performance takes you west over the low spot at Ranchita, heading toward Lake Henshaw before turning southwest. They will generally not have transponders, will not generally be visible on ADS-B and can be from very low altitude up to 18,000 ft.
When better times for eating out return, call Casa del Zorro and they’ll send a van to get you for lunch. The airport restaurant closed again a while back, it comes and goes.
I might have flown to Borrego Valley a time or two... Like everybody else I’ve ever met in person I’ve never used FF to do so, it’s a local milk run that after a while you can almost do with your eyes closed.
