If at all possible ask in person or in some face-to-face way. You'll want to develop a human connection with this person and that is best done without email.
There are a number of ways to approach getting help.
One way is to do some work (or at least get started on it) and then ask this person to take a look at it with you and give you some advice or their point of view. Just by seeing what you've done, a skilled and empathetic practitioner will be able to assess your skill level and be able to adjust their communication to meet you where you are at.
Another approach is to ask this person to show you some of the stuff they've done. Basically have them give you a tour of a project they're working on so you can see how it's put together, what their workflow is and how they deal with changes. All of that will stimulate questions you may want to ask. Most people are proud of their work, so this is a easy way to get someone to talk productively in a way that helps you.
Keep in mind that people have widely different attitudes about mentorship and learning, especially in computer-related fields. The fact that you're concerned about wasting time is good. Not everyone is generous with their time and someone who is extremely skilled might not at all be good at mentoring (those are two very different talents).
Moreover, the workplace itself might limit what is possible. I know what "Backlog" is but the very origin of that word suggests an environment where workers are seen as "behind" before they even start a project. If the job is a pressure cooker environment, it's yet another obstacle.
Whatever the situation, you'll want to be strategic about how you approach the more senior worker. You certainly don't want to engage them outside of work hours unless they specifically want to do that.