If you're going to change and track something to test its correlation to weight, please consider calorie intake instead.
10k steps is really not going to burn many calories (my marathon running friends use the "1 mile = 100 calories" rule of thumb). Additionally, many people starting cardio see an increase in appetite and if they are not already used to carefully controlling their food intake, end up _gaining_ weight.
Having known many people who have made the 50 lbs. fat loss goal you're looking at (and having cut 25 lbs. myself), a major diet change is likely the only thing that'll get you there. If you continue to take in enough food to sustain yourself at ~200 lbs, nothing short of training like Michael Phelps will get you down to ~150 lbs.
EDITED to correct some bad arithmetic :(
Weight also impacts how many calories are burned. (http://caloriesburnedrunning.org/; not sure how accurate this particular site is, but I've seen similar results from other sites). A 6'6" person with a healthy BMI weight of 215 lbs would burn around 175 calories per mile at an 11.5 minute mile pace.
Despite this, as a tall person who runs a lot, I've found that a diet change is still really important if one wants to lose weight. (But after this is done, it is nice to go for a ten mile run on the weekend to cut a half pound that can be used as weight loss or to allow more eating throughout the rest of the week.)
I once dropped 50 lbs in about three months solely from returning to my previous running habit. At least according to my calculations, I was burning closer to 1000kcal/hour (more like 100kcal/km). I wasn't doing competitive-style twice a day training, either. Just one run, six days a week, usually for 60-90 mins at a BPM of <150. Once a week I ran 800m sprint repeats all-out, and two or three times a month I did a long run (which was increasing by about 10 minutes each time until I got it up to 4 hours).
All in all it averaged out to just over an hour of exercise per day, and the mental benefits more than made up that time in productivity. I didn't change my diet at all, except to eat a bit more after the long runs and possibly to eat a bit more cereal. And I lost just about every spare bit of fat on my body.