public class DoubleSpeedAnimation : DoubleAnimation
	{

		public double? Speed
		{
			get { return (double?)GetValue(SpeedProperty); }
			set { SetValue(SpeedProperty, value); }
		}

		// Using a DependencyProperty as the backing store for Speed.  This enables animation, styling, binding, etc...
		public static readonly DependencyProperty SpeedProperty =
			DependencyProperty.Register("Speed", typeof(double?), typeof(DoubleSpeedAnimation),
				new PropertyMetadata(null));

		protected override void OnChanged()
		{
			base.OnChanged();

			if (From == null)
			{
				string target = Storyboard.GetTargetName(this);
				PropertyPath property = Storyboard.GetTargetProperty(this);
				if (target != null && property != null)
					BindingOperations.SetBinding(this, FromProperty, new Binding() { ElementName = target, Path = property });
			}

			if (From != null && To != null && Speed != null && Speed != 0.0)
				Duration = TimeSpan.FromSeconds(Math.Abs((To.Value - From.Value) / Speed.Value));

		}

	}

    <Grid Width="800" Height="600">
        <Grid.Triggers>
            <EventTrigger RoutedEvent="Button.Click" SourceName="clear">
                <StopStoryboard BeginStoryboardName="doubleAn"/>
                <StopStoryboard BeginStoryboardName="speedAn"/>
            </EventTrigger>
            <EventTrigger RoutedEvent="Button.Click" SourceName="double">
                <BeginStoryboard x:Name="doubleAn">
                    <Storyboard>
                        <DoubleAnimation
                            To="700"
                            Duration="0:0:10"
                            Storyboard.TargetName="border"
                            Storyboard.TargetProperty="Width"/>
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
            <EventTrigger RoutedEvent="Button.Click" SourceName="speed">
                <BeginStoryboard x:Name="speedAn">
                    <Storyboard>
                        <local:DoubleSpeedAnimation
                            To="700"
                            Speed="70"
                            Storyboard.TargetName="border"
                            Storyboard.TargetProperty="Width"/>
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>

        </Grid.Triggers>
        <Border x:Name="border" Background="Green" Width="50" Height="50" VerticalAlignment="Top" HorizontalAlignment="Left"/>
        <Button x:Name="double" Content="DoubleAnimation" VerticalAlignment="Bottom" HorizontalAlignment="Left"
                Margin="10" Padding="20,5"/>
        <Button x:Name="speed" Content="SpeedConstAnimation" VerticalAlignment="Bottom" HorizontalAlignment="Center"
                Margin="10" Padding="20,5"/>
        <Button x:Name="clear" Content="Go to Begin" VerticalAlignment="Bottom" HorizontalAlignment="Right"
                Margin="10" Padding="20,5"/>
    </Grid>
</Window>