First of all, happy to hear you got the passion to ride back.
The quick answer to “ Doing blocks or just low heart rate rides?” is: both. They both provide benefits. Blocks/intensity provides training stimulus to increase your power over a broad range of efforts. Low heart rate rides provide some recovery and training effect as well as build base.
The more specific answer depends on what your goal/goals are. A FTP of 300W (315W x 0.95) is a great base to continue from regardless.
Are you interested in continuing to compete? Road racing, cyclocross, MTB, gravel (fondos), time trials/triathlon? Short intense events (crits) or longer, multi-hour efforts? Or just a general improvement over all disciplines? As you can see, it is a wide open arena. Focusing your training to your interest can be motivational and effective. And the training for one discipline can be vastly different from another.
This day in age, there are plenty of training tools that are easy to access and can provide tried-and-true methods to obtain improvements. There are several indoor training tools (TrainerRoad, TrainingPeaks, The Sufferfest/Wahoo SYSTM, Zwift, etc.) that allow controlled training regimens and all of these have access to structured workouts and multi-week/month training plans to structure your training to build your fitness to your specific goals, and even target specific events to peak for if you are interested. Some of the some of the workouts can be done outdoors as well (for example, longer endurance/base rides) are much more pleasant outdoors if the weather is cooperative. One more benefit to these indoor training plans/programs is that each workout is geared to your ability/FTP. The harder workouts will leave you completely spent, but the benefits are reaped after some recovery and repetition.
A very simple "plan" is a three-week build, with the fourth week easy for recovery. Typically you have 2-4 days of short work (45-90 minutes each) during the week from Monday to Friday or Tuesday to Friday (Monday is always a rest day). These are usually harder, more intense workouts, intervals with recovery between hard efforts. But it is important to mix it up. Maybe one day with two hard 20 minute intervals (2x20s) and another day with shorter but harder intervals 15 times 1 minute intervals (at about 1.25 times your FTP) with 1-minute recoveries at 0.4 x your FTP. Then on the weekend, one or two longer rides at an endurance pace. Three weeks of the hard stuff, then on week four, maybe just two workouts during the week, not as hard as the previous weeks, and one moderate length ride on the weekend. Then just repeat the four-week cycle again.
- Mix it up. Variety keeps you motivated.
- Recovery is important. Overdo it or work a recovery week too hard and you will not be as fresh for the next hard three-week block.
- Get a goal, or goals to work towards. A goal is a target to aim for, and is also a motivating focus.
- A structured training plan is almost guaranteed success. Without a plan you are just riding for fun. Riding for fun is ok, and it is nice to break from the rigor and rigidity of a plan, but if you are focusing on a goal or a key event, a training plan of some sort is where it is at.