Just Got a Road Bike, After A Week Cycling Got Harder?

Hi I come from riding a mountain bike for most of my life, but when I decided to get a new bike I wanted to try out a road bike. I am pretty overweight and unfit so I thought it could help me lose weight. At first I started biking around 10 miles a day which was a lot for me but was not tiring. After doing that everyday for a week I started getting tired. From barely biking a mile I am sweating a storm and can barely breathe, can someone explain what is going on? When I ride my mountain bike everything is fine, I can go even farther than 10 miles, why is it that I can't ride my road bike? If it helps it is a used Sciwinn Phocus 1400.
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I think a good solution for you is to be progressive. By that i mean ride 3-4 times a week and gradually add km's to your weekly total to build up your base fitness. Maybe 10km per week? Also make sure you are rested before a ride. I always have a rest day between rides and only ride when i feel like it. I never ride 2 days in a row.
Grab a heart rate monitor and the Stravistix plugin for Google Chrome along with Strava if you're not already on there. Use the the HRM for all your rides and Stravistix's "multisport fitness trend" will give you an indication of when you are over-doing it.
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