First of all welcome to Sparkpeople - this is a fantastic website with lots of great advice on healthy weight loss and exercise, plus the benefit of great support from fellow 'sparkers' so I am sure you will do great here.
The 70lb weight loss has obviously given you a jump start but, as others have mentioned, this is not a healthy approach. The main focus of Sparkpeople is more 'healthy living' than 'diet', so you won't lose weight as fast as you did before but you will still lose weight and more importantly, improve your health.
Tracking your calories is an excellent plan and if you follow the sparkpeople set-up it will show you how to do that and give you a personalised calorie target based on your current weight and age. You definitely won't gain going from 600 to 1200 calories because 1200 is still less than your body needs each day to maintain a stable weight.
Adding in exercise to your program will really give your weight loss a boost, start out slowly, walking or cycling is great - just 10 minutes a day to start with and aim to build up from there. In terms of pace, that is a personal thing, you need to aim for a pace which is easy enough that you can sustain for 10 minutes initially while equally being hard enough that it raises your heart rate and leaves you feeling warm at the end of it.
There are lots of great articles and advice all over the site so take some time to browse and don't hesitate to ask more questions if you get stuck - there's always someone to help!
Very best of luck!
| current weight: 171.0