Marathon Training Plan (Increase Pace)

hi! I am running my first half marathon this weekend my typical pace for longer runs is 10:00-10:20 and my hr lingers around 160s. I have noticed in my last few runs that I can run at like 9:45 with a similar heart rate. I signed up for this race at the end of july which is when I started running so I just followed a plan from google images lol. I end up with around 20 -22 miles per week. I am planning to run the Houston Marathon in January and I want to get under 4 hours maybe even 3 hours 45 minutes. I have never incorporated any XT or interval /tempo runs, I have just been logging miles. I am looking for a plan/coach that with help me be more organized to help me achieve my goals for the marathon. Does anyone have any recs for the best online plans or ways to find a coach?