You can do everything, but you can't do everything at the same time. Words to live by. If you don't have the capacity to deliver an acceptable level of service and attention, turn business down or delay it until you open up more capacity. It's a problem solos deal with all the time.
You do legal work. You do the work of managing your practice. You have to know how much time you need for both, and what that translates to in terms of your availability. Taking business you can't handle is the worst client service move you can make. You'll become someone who is afraid of phone and email inquiries from clients that are on stalled on the runway. You're at risk for delivering work that doesn't represent your capabilities. You are at real risk of missing deadlines. Reputational risk rises with every day you can't meet your commitments.
The time to be organized and create the processes that support your growing business is BEFORE you are busy. Even if your practice is a part-time one, you should manage tightly. Build an environment with an eye towards the future. Processes, templates, checklists, formats. Estimate, plan, and review your estimating processes to make them better. Stay connected to your calendar- not just in terms of deadlines, but in terms of the actual work hours you need to deliver client work and manage your firm. Know how far into the future your current obligations have you booked.
The first approach to leverage is simply to leverage yourself with processes and technology. Then comes outsourcing and/or hiring. If you plan on earning a solid income, you will be adding resources. From Day 1, designate a budget you save for that purpose. In the beginning, maybe it covers a phone or transcription service. Later, maybe a contract lawyer, paralegal, virtual assistant, associate, etc. comes into the picture. It is a delicate dance, adding resources vs. overextending. Don't fear it, get ready for it.
I get the fear thing. It's normal to be afraid to miss opportunities. Normal and even responsible to be concerned about the expense and management time that adding resources involves. However, the moment you have one client that wishes you had turned him down because you're clearly too busy to deliver on your promise- it's not only that client's future business but his future referrals that you've put at risk. I want you to be wildly successful. I also want you to be wildly responsible. Don't take business you can't handle, delay it if you can and turn it down if you must. Trust me- if you're managing your future opportunities, better to NOT take the business than disappoint a client. A lot less stressful for everyone, to boot.
Life is short, struggle is optional. Get help just before you need it. Say "no, thank you, I am so sorry" and refer someone when you have to. Manage responsibly.