Marathon time calculates your estimated finish time for a full marathon in real-time. This is a great tool when striving for a new personal goal. Include this App in your running mode and use it pace yourself to reach your goal.
Suunto Ambit Collection
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If it's only calculating with your Avg speed it's not really that interesting. Maybe people would be more interested in knowing what the minimum speed for the rest of the race needs to be to reach their goal. For example: your goal is 3h and you still need to run 10k and you're already running 2h15; so you need to run the rest in 4'30"/km or faster.
Everything every runner always wanted to have:
1) Four estimates (5k, 10k, Half, Full Marathon) in a singe APP: http://www.movescount.com/apps/app10012615-MarathonhalfM10k5k_time_estimator_in1
2) Riedel's formula: http://www.movescount.com/apps/app10016363-Marathon_Time_Estimates_Riedel_and_Speed
3) Cooper Test: http://www.movescount.com/apps/app10013191-Cooper_Test_estimate_HR_time_left
4) Cardiac Drift: http://www.movescount.com/apps/app10012137-4in1_Interval_Training_Cardiac_Drift_oth
I don't like this app - the estimate is based on the AVG_SPD for the whole distance covered so far. It is rather difficult (and not recommended) to keep a steady speed during the whole marathon (see, e.g.: http://www.runnersworld.com/race-training/marathon-pace-perfect) and usually you will run the 1st half a bit faster than the 2nd one. That means that the app by Movescount will be showing you too optimistic results.
As a prep for marathon try interval training with this APP: http://www.movescount.com/apps/app10012137-4in1_Interval_Training_Cardiac_Drift_oth - it's a multi-APP - you have Cardiac Drift (comparison of performance, i.e. the ratio SPEED/HR between intervals), Karvonen HR%, max HR, max Speed in one app. And also timers for interval and rest periods.
This value doesn't tell me anything:
what'is the difference between running in 12 km/h during 42 km and the estimated time: 3:30'58? (yes there is not difference) 2 points of view to show the same.
If I can see the real time... I mean the expected time to finish the marathon that should show me somethingh that make sense: I begin to run at 12:00 and depending of the average pace I'm running the estimated time to reach my goal is:
15:30 or 16:00 and I can compare this with the real time :)