Mountains to Sea Trail from Newport Beach is a 12.2 mile moderately trafficked point-to-point trail located near Newport Beach, CA that offers the chance to see wildlife and is rated as moderate. The trail offers a number of activity options and is accessible year-round.
Very nice and safe trail from Irvine to balboa beach.
The Mountains to Sea trail is an all-day hike or bike ride (it's MORE than an all-day hike if you're in a physical condition best described as "perfect for watching TV"). It winds from the mountains (duh) to Newport Beach, and includes parts of different trails, including a beautiful part of the Back Bay trail in Newport Beach. Some of the parts of the trail are beautiful, some are following a big concrete ditch under roads and freeways. It's a great way to get in shape, given the rising and falling landscape. I'll be the first to admit that I'm not keen on changing my shape; it took me 40 years to end up looking like this, and I'm happy with it. I've made my peace with it. This is a shared trail; you'll meet bikes, horses, walkers, joggers, people wandering around lost, and people walking their dogs. Watch out for the bikers; they're generally kinda rude and it seems that they think their tight goofy-looking pants entitle them to trail ownership (kind of the same way heavyset people in spandex think they own the whole trailer park). The difficulty rating changes; it can be easy or moderate depending on where you are. I'm marking this one as moderate mostly due to the length. There is virtually no tree covering, so make sure you've got sunscreen and a hat. A good pair of walking shoes is a must if you're going to tackle the whole trail.
The Mountains to Sea trail is a big part of OC. It starts in Newport Beach, winds through Irvine, and some parts of Costa Mesa as well. Depending on your skill set, you can tackle different portions of the trail as they are split in sections based on intensity.
I have walked or ran every inch of this trail since 2007 and enjoyed every aspect. It's great to work on hill training for marathons or practice speed runs at the Back Bay portion that starts in Newport Beach near the 73/405 freeway at Jamboree.
The trail is shared by bikers and some will be courteous however others will provide no warning so be wary. In other respects, since the trail runs through various cities, it is very scenic and always fun.