heya... i think you should try a heart rate monitor, i think you must be counting your HR wrong. your profile says you were born in 1980, you're turning 26 in november, right? theoretically, you're max heart rate is 220 - age = 195 bpm (for a 25 year old). for you to have a heart rate as high as 240, well, that would signifiy a heart problem. at that speed, you're heart would not be able to pump blood efficiently (it would be beating so fast it wouldn't have time to refill between beats).
also, your target heart rate should not be at 180. that would be 92% of your theoretical maxHR. unless you were a competitive athlete, you would never try to get your HR that high. 85% maybe, from time to time, but not as a daily workout.
so first off, i'd definatly recommend getting a HR monitor. it will make monitoring your HR much easier (get teh kind with a strap, not the type that let you hold yoru finger and thumb to it to read your pulse), you can get the simple ones for as little as $50.
www.fwonline.com is a good store. if you're willing to spend the money, get the polar monitor that comes with the chest strap that allows you to replace the battery. it'll save you money in the long run.
also, depending on your fitnses level, you'll probalby want to exercise at between 65-75% of your heart rate reserve. and maybe even higher intermittently. you can figure out your HRR by taking your theoretical max (unless you do an exercise stress test which can tell you your real HRmax), anyways, the equation is...
maxHR - restingHR = reserveHR.
multiply reserve HR by the upper limit % (likely 75%)
multiply reserve HR by lower limit % (likely 65%)
Add the reserveHR to both limitHR's and you'll have the range you should be working in. but choose percents based on your fitness level. the more fit, the higher they can be.