Glenn,
Keep in mind your heart rate is going to go up in this heat and humidity. There isn't much you can do about that so try your best not to be overly concerned with your heart rate seeming to go up. If your pace seems stuck consider doing 5-6x100m strides after every other run throughout the week on top of your regular training. The strides are too short to do any damage and too minimal in distance to build up any lactic acid. One thing I HIGHLY suggest you do is if you have access to a physician consider getting your glutathione, iron and ferritin levels checked. Hardly anyone knows anything about glutathione but it is an antioxidant that every cell in our body produces. We all can easily become fatigued if we run low yet we unknowing that fact runners think they are overtrained or their best days are behind them which is untrue. It also detoxifies free radicals and other toxins that build up in our body. As athletes we know how to train physically but this is a crucial aspect of nutrition that can make or break endurance athletes. I am a 2.19 marathoner and know what I'm talking about and know you have much faster times ahead of you if you improve these molecular areas of your body. I am a max international associate and take this aspect of sharing glutathione and riboceine very seriously with those I coach and answering questions for other runners and non-runners alike.
What do you need to do? First, be patient. It takes approximately 21 days for the body to adapt to any stress load you place on it so what you do today in terms of workouts will not truly begin to surface in a positive light until 3 weeks down the road. You have to lengthen your long runs and most importantly the duration you spend at or near your goal race pace. Adding in strides at the end of your easy runs, over time, means your spending MILES at sprint speed ON TOP OF your regular training. They duration of the sprints are too short to build up any lactic acid but over time your body will supercompensate and grow back stronger for adding these in.
You want the body to use fat at race pace and conserve carbohydrate stores and that can only be done by higher anerobic training. Easy runs are great for recovery but when it comes to a specific goal pace in mind you have to train in a specific way. If you are early in your training phase consider doing very short fartleks at very high intensity with longer recoveries i.e. 10x30 seconds very hard followed by 2 minutes jog between each burst. Once you do this, over time, the stronger you are going to get, faster your paces are going to be and the longer you are going to be able to run.
Hope this helps,
Nathan