Training on heart rate

When I first started training for Ironman in 2010 I relied heavily on my heart rate monitor.  Every bike and run session was based on my heart rate.  However by 2012 I had moved away from heart rate and went to time based targets.  After all I could tell what my heart rate was without the monitor.  Last year I was using it again but the readings from the garmin were so erratic it was a waste of time.  But recently I've been struggling controlling my heart rate so I bought a new heart rate strap and it's solved the problem.  Now I've got accurate results although I don't use it in the pool.
After 2 weeks out of the pool, for various reasons, I went back this week and had two good sessions. Swimming provides me with a number of benefits; it eases my legs after big miles at the weekend, it's good for my breathing, it's a good workout and, as I have a 10km swim In September - it's necessary.
On Saturday I went out for 15 miles (3 laps) trail running on my own.  My max heart rate based on age is 168 (I know this isn't accurate but it's a starting point).  The previous weekend I was constantly at 170 and eventually hit the wall.  So I thought I'd try and stay between 135-150 and if I got to 158 I'd walk until it reduced.  The undulating terrain at Chatelherault (425 ft of assent in every lap) means your heart rate will vary but the longer you run, the more your heart rate increases.  So by lap 3 I was doing a bit more walking on the hills.  My average km per min time was 6.29.  In reality this meant I was 2 mins per lap slower than normal but I could live with that.
Today I had company and the guys stayed at my pace. 4 laps meant the average time slipped to 6.38 per km however I need to remind myself that completing 53 miles across tougher terrain will be at a slower pace.  3hrs 25 mins won't compare to 12-13 hours.  All I am doing is building the miles and doing back to back long sessions.  I won't be going over 30 miles before April 26th in one go, it's more about preparing my legs and my mind.  Again my nutrition was right but I need more fluids onboard as I am dehydrated after 20 miles. 
For me the training on heart rate is working for me.  It's slowing me down and making sure I am not exhausting myself.  It's fantastic having a stretching goal to aim at - I'm loving the miles!