I was born and raised in Escondido (but now live in Carlsbad). You’ll be just fine for training, believe me.
For swimming, there is a YMCA in town (http://palomar.ymca.org/english/). Join up and you’ll have plenty of swimming opportunities. If you want a nicer pool, swim down the road in Poway (http://www.ci.poway.ca.us/Index.aspx?page=355) or do Masters in Carlsbad (http://www.carlsbadmasters.com/). You could easily ride to either pool. Both are 15-20 miles away. Also, if you don’t have a car, just get a buss pass for NCTD and you’ll be fine. You can get anywhere you need to go, and you can bring your bike with you on the bus.
For cycling, you’ve got access to some of the best cycling in the world. I grew up racing road bikes here, and there are tons of good athletes in the area. If you want to ride hard, meet up with the Ranchos crew on Wednesdays and also their other training days (http://www.ranchoscycling.org/). If you don’t mind riding to/from the group rides, there are also the Swamis rides on Saturday and Wednesday (about 20 miles away in Solana Beach and Oceanside), SDBC rides (Saturday and Sunday, about 25 miles away in La Jolla), TCSD rides early on Tuesdays and Thursdays (20 miles away in Solana Beach), and so on. For solo riding, you can head east out toward Valley Center and Palomar Mountain, Julian, etc. (big hills), or north up through Highway 395 toward miles and mile of roads (rollers and some big hills), or west toward the coast for plenty of riding (fairly flat). Riding south is a bit complicated until you learn the area a little better, but there are tons of options there, too (RB, Poway, down into San Diego, etc.).
For running, there are lots of trails at Lake Hodges (http://www.lakehodges50k.com/), and several neighborhoods have dirt trails next to the sidewalks that are good for running. My favorite sidewalk trail is the one off Country Club and Broadway in North Escondido. 2.5 mile loop, flat and fast. It’s easy to get in 7-8 miles by running loops around that. In general, you can run anywhere in town, though. Pretty much every road has a sidewalk.
In brief, yes, Escondido is a very sensible place to live and train while you are here. The weather is always good and the training opportunities are abundant. Let me know if you have other questions. I lived in Escondido for 27 years, so I know a lot about the area. Also, I’ll be doing Cali Half, so I’ll see you there!