Best way to be lean is run, run, and more run. You can do your situps before you run but know that its been proven crunches do more help. put your hands on your shoulders X'ed across your chest and push your chin straight up (not towards your knees) to the sky. Place one ankle on your knee from the normal situp position and do crunches like that then switch legs and do it again usually about 20 count each in a row (3 part exercise) without resting. Then normal crunches 20 count as well. It is a three part exercise and alternating your legs like that will get you that V cut you want. Do 3 sets. I also recommend doing 25 pushups atleast. No woman wants muscular chest or w/e but you definately dont want flabby arms so that will tone your arms.) Women tend to love running because it makes for sexy legs and burns all fat without bulk muscle. Best way to start running is go to a college or official size track and commit to 2 miles which is 6-8 laps. Run 1 lap, walk a lap, run 1 lap, walk a lap,.... Until you hit 2 miles then your done. After the first week or second start attempting to run more of the in between laps until you fill the gaps completely at your own pace then once your running a solid steady 2 mile add another mile. Then every month add another if you like. You could get up to running 5 miles in 45 minutes (standard for most military people women and men) and I assure you you will be lean and averagely healthy.
If you want to get real lean cut you can mix sprints and shuttle runs. One way to better run times and burn calories is where you walk a lap, jog a lap, and then full on sprint an entire lap. Do this 3-4 times.
I suggest you alternate distance one day then speed workout the next. Take like sunday off and give yourself one full days worth of rest. Your diet should consist of ramen noodles and baked chicken breast and all you can eat vegetables (steamed or boiled) no frying(dont cheat yourself. Diet is important if you want fast gains in your performance and appearance. just remember EVERY CALORIE YOU EAT IS ANOTHER MINUTE ON THE TRACK so unless you wanna bust yur hump constantly ask your self is that candy bar/ pizza worth another hour on the track? And say goodbye to Soda and anything with High fructose corn syrup in it. Drink plenty of water. If you hate water get crystal lite packets or something to put in it. Eat oranges for snacks (citrus kickstarts and aids in digestion and metabolism)
United States Army Infantryman (7years of service), Have had several personal trainers in my day.