I think the key points about exercise are that it should be regular, and you need to be able to keep doing it long(ish) term. For me, the most successful approach has been to incorporate exercise into my daily commute. At the moment, I work in central London, so walk from home to tube and from tube to work each morning, and back again in the evening. That isn't a great deal, but it had a noticeable effect on my blood pressure.
That is, until I broke my leg - that really has spoiled things.