Recently I've been enjoying climbing with my bicycle*. There are a few hills, namely Fedex Drive and Thorn Run Road, that are close to my house and provide a decent (9 - 13%) extended grade challenge. On these, if I go 3mph, my heart rate typically settles in around 155bpm but at 4mph it gets up to around 165bpm. I can't really get much over 172bpm for any extended period of time (especially at my age) as I start feeling a bit too close to the tolerance level
Hills are incredibly strong -- yet are always amazingly so silent. There is an addictive quality here, that is both exhilarating and humbling.
I'll never wear a polka-dot jersey -- but in my own Walter Mitty fashion, I will.