How should I increase my training to minimise the risk say of a heart attack?

As part of my current exercise programme I jog until my heart rate goes up to 140 bpm then walk till I have recovered and then jog again and so on for about 30 mins.

I am anxious about pushing myself any harder than this. My age is 47 and 140 bpm already representents a shade over 80% of my max heart rate. Applying the 10% rule or even just increasing my training heart rate by 5 beat per minute seems risky to me.

By the way my BMI is 30 and my blood pressure is controlled with 5mg Amlodophine daily. I have lost 20 pounds over the last six month mainly through walking 30 to 60 minutes 4 or 5 times per week and sticking to a daily 2000 Kcal diet. My average morning blood pressure reading reading is about 130/90 at 60 bpm and 3 to 4 hours after taking my tablet at 6am my bp is usually around 125/85 at 70 bpm. My aim is to lose another 30 pounds or so to get my BMI down to 25.

Christopher Anastasi