Personally, I think heart rate training is overrated and often very inaccurate. As an alternative, train by your breathing.
- If you can barely catch your breath and are in immediate danger of collapse, you're way-way over extended.
- If you are breathing hard but are able to sustain the level of exercise for many minutes on end, you're probably at your sustainable training limit and, unless you're training for competition, you should slow down.
- If you are able to carry on a conversation in short sentences during your training, then you are working out at a sustainable pace.