I ride alone the vast majority of the time. One of the key considerations I use when considering the risk of riding alone, is traffic volume. If you're riding a somewhat popular trail during the day, there's little added risk to being alone, because someone will be coming by within a few minutes anyway.

OTOH, if you're riding a very secluded, or rarely used trail, or are the last person on it late in the afternoon, it can be hours or until the next day or even a week before anybody happens by, and you have to be ready to manage 100% solo. This isn't to say that riding lonely places solo is stupid and shouldn't be done, only that you have to factor likelihood and consequences into the decision.

These days, they make personal emergency locator beacons, similar to those used at sea for years, so that's an option if riding in isolation. Also, if there's a ranger or park gate attendant, let them know where you'll be riding, and most important --- please --- tell them when you're leaving.