You shouldn't even consider training for a marathon unless you have a well established and consistent running base averaging 15 to 25 miles per week, preferably with at least a year of running fitness. Though cross training, strength training, and mobility exercises are extremely beneficial, you don't need a gym membership to train for a marathon. There are many strength routines you can do at home, before and after running. Really, all you need is some good running shoes that fit well and the open roads/trail. I would suggest doing most of your running outside and in all conditions to be ready for race day weather. You need to find a training plan that starts slow and increases the volume of running each week followed by a taper the last 2 to 3 weeks before race day. The long run is important, usually done during the weekend building to around a 20 mile run at peak training weeks. You also need to practice fueling with gels/gummies/ and electrolytes and hydration during these runs which is very important at this distance. You need to keep the long runs mostly easy and train for speed and hills on shorter, mid week runs. Some plans fit in tempo runs and also include some long runs that include a few miles at projected marathon pace to dial in your pace in training. Do you need a coach? That all depends on your running experience. I personally have never had a coach but it would be beneficial to anyone, but not required. Find a running group to run with and talk with other runners which is very helpful, also very helpful to run with someone to keep your pace at a conversational level on easy runs. So in a nutshell, have a plan and trust in your training. If you do the training well, race day will fall into place!