Thank you so much for your question.
There are two incredibly important angles I need to throw your way. The first is about time management, to be expected. The second is about lead generation/customer retention.
I agree with John about the urgent vs important. However, I don't think we can stress enough that focusing on important tasks is critical. You've probably heard of the 80/20 rule. 80% of your results come from 20% of your effort and time. This is true. So, how do we fill up an entire workday, then, and still feel like nothing was accomplished?
Seemingly urgent tasks. Things like email, phone calls, and meetings come to mind. I disagree that you need to be taking every phone call. Somebody should be, but it doesn't have to be you. And hey, if you can't get to the phone right away because you're vastly improving the UI on the software I license from you, then I'll be happy if you get back to me tomorrow. Be sure to explain this to me, your hypothetical customer, however.
Regarding time management tactics. Schedule everything. I fought this for a long time, but finally giving in to this advice has made a world of difference for me. It prevents thinking about your next task. If it's already written down, just do it, and ignore those pesky emails until a specified time, for a set duration.
Next, customer retention.
If you already have customers, I would work on making them ecstatic about your product and/or service. Spending 10 hours on lead generation is wasteful. Use that time, instead, to interact with your current users, making the product better for them. If somebody inquires about your business, of course, treat them with the same level of attention, but don't go searching for new business.
Focus on making your current clients so happy that they will spread the word about your services.
Gary Vaynerchuk on caring your face off
The Thank You Economy by Gary Vaynerchuk
The 4-hour Work Week by Tim Ferris